Andy, in an update to previous comments....
I carefully tested the 40-150 PRO with the 1.4X TC against the 75-300 this morning by setting the FL of the 75-300 to 200mm.
I let the camera choose shutter speed, and aperture with the ISO fixed at 200 and no filter on either camera.
As expected, the 40-150 shot 1/500 and f/7.1 while the 75-300 shot 1/250 at f/8. I have to say the sky was a bit brighter when I took the photo with the 75-300 30 minutes to an hour later. One stop maybe? That would drop the shutter on the 75-300 image to maybe 1/160 and the aperture to f/7.1 - just a guess Andy. I can't measure the amount of light. I shot homes 3.5 miles across a lake with mountains another 60-70 miles behind the homes, a good example of a long FL compressed landscape with the homes more in focus than mountains. Need focus stacking to focus both and don't have it. I viewed the homes at 100% crop to test the lenses.
I knew what the approximate difference in aperture and shutter speed would be. I expected the PRO lens to be sharper.
More light might have helped the 75-300, however inspecting the images at 100% zoom, I found so little difference in sharpness I don't think anybody could see it without 100% cropping. That surprised me. I can't honestly say the PRO lens is sharper at its extreme FL with the 1.4X TC compared with the middle of the 75-300 range. I've observed slightly sharper images with the PRO lens when the TC is not used. It isn't enough to make me want to leave it off. The images are detailed enough for me with the TC and with the 75-300.
I do notice the PRO lens seems to control glare better, but it did have less of it to contend with. In equal light or with a CPL I may not see a difference, but keep in mind the difference in the amount of light showed up as f/7.1 to f/8 with the 75-300 choosing f/8 and the shutter difference of 1/250 to 1/500. I'm not sure if this difference in light tilts sharpness to the 75-300. I expected the PRO lens to be significantly sharper. It isn't.
What I am sure of, is compared to the cost and results, the 75-300 hits above its weight class at MSRP $550. I think its an underrated lens. I paid $350 on the OLY refurbish page for this lens I consider to be as-new. I think these are demo lenses, not sold to the public and returned for refurbishing. Keep in mind the 75-300 is not weather sealed. I wouldn't take it out in rain or snow. It is much smaller and half the weight.
I didn't test the 14-150 which I also have, but I used this lens a lot in the summer, and I am surprised at the quality of the images I have from it, even though its a 10X plus lens. Some images are close enough in quality to the 75-300 that I can't say the bigger lens is much better, if better.
From this decidedly unscientific, but real world test, I conclude, at least for myself, that as long as I can keep the ISO down, I am happy with the images from any of these three lenses, so the only time I need the PRO lens is when ISO rises above 400, maybe to 640 using the 14-300 or 75-300.
I do a lot of motorsports photography. In that world, I pan a lot at shutter speeds between 1/80 and 1/160, so keeping the ISO as base is no problem. The problem can be preventing the lens from stopping down past f/11 in bright sun where I might encounter diffraction.
When a car is pointing at me off a corner or down the straight, I like 1/250 - 1/800 depending on distance and speed of the car, and angle to the camera. In gloomy weather, fog, shade I like to shoot in, this is where I need the PRO lens to keep the ISO at 200. I do not shoot in these conditions that much, but it results in compelling images with no hot spots on the cars, and limited shadows to bring the image to a dynamic range the sensor can manage, so even for a few shots a day, owning the PRO lens is worth it for me.
I know the specs and consult the lab test data, but for me, a decision is only valid if its made using results in conditions I shoot in. I suggest you take this into consideration before buying more lenses. If you look at the photos you've taken, it will show you how often you need PRO glass to keep the ISO down to 200, 400, whatever you think is acceptable.
I would be willing to bet, as long as you can shoot the same ISO with all three lenses, the delta in IQ, all other things being equal, you will not see enough difference between them to justify spending $1,850 for the PRO lens and the TC. Maybe at 100% crop. Maybe not. It then becomes a decision of the additional cost for those shots where the ISO rises to 800+ if it does make a difference that matters to you. Only you can know this by reviewing the images you saved.
When you hike, you will appreciate a 10-13oz lens in your pack, rather than a bigger 1.67lbs lens and a 6oz TC to hump around with.
I know I'm running the risk here of emotional responses, maybe outrage for suggesting the lab tests and the quality and cost of the PRO lens suggests I am wrong about this, or the "consumer grade" lenses really are better than the test data and price suggests.
Either may be true. I can only tell you my experience with these lenses. Your experience may lead you and anybody else to a different conclusion which for me, is inside the light window of the 14-150 and 75-300, when you can keep the ISO at 200-400, maybe a little more, for monitors and prints, you will probably do just as well with the consumer grade lenses as the PRO lens, and you can buy both of them for less than $1,000 if you shop. The 40-150 PRO is an MSRP$1,500 lens vs. $1,150 for both of the other lenses and they cover a bigger range - 14-300mm vs. 40-210 with a $350 TC = $1,850 MSRP.
I bought the PRO lens because I need it for the FL in low light. You may not need it. You may do just as well without it. My test is not scientific, but for me its valid because its actual photography in light I shoot in, and a composition I shoot as a real world test. This is my suggestion to you. Look at the data on the images you have, to decide if you need f/2.8-4 glass and how often.
For what its worth, here are the test images. Took them in RAW, converted with OLY viewer 3, sharpened a little. I played around with color a little so ignore that. I suggest you just look at sharpness and control of glare in crops. Glare is controllable with a CPL filter. For me, the image taken with the 75-300 without cropping, is for all my intents and purposes just as good as the PRO image. The 75-300 gives you another 180mm of reach if you need it. I could take a better photo of an individual house with the 75-300 because I wouldn't have to crop as much and the sharpness between the two lenses is pretty close at these apertures in this light, though the 75-300 might be as sharp at 300mm as it is at 200mm.
The homes are 3.5 miles away per GOOGLE maps. I probably shouldn't have played around with the color. Too late for that. You can see one image had more light. The sun had risen more and is reflecting off the water in front of the homes. This may account for 100% of the difference in glare and increase the difference in shutter speed and aperture. I think they are pretty similar in conditions and result. I am just as satisfied with the image made with the 75-300 as I am with the PRO lens - for $950 lower MSRP. I hope this is useful. It was for me. Without the glare off the water, the camera setting would have been probably a stop further apart, but the image quality may have been dead even.
