Telephoto for landscapes - 40-150mm f2.8 or something else?

I like the 40-150 but on IQ I disagree. Cropped 100% there is no contest. The PRO is WAY better and you can buy a 1.4XTC for it and get out to 420mm. It is also weather sealed.

I went to the PRO lens because sometimes the light falls and I have to boost the ISO too much when I need to shoot 1/800 and higher. The images lose too much detail, are too noisy for me.

I like shooting in low light for reduced shadows and glare. If you shoot a landscape with the 40-150 at a slow shutter speed it works, but if you need a fast shutter speed it doesn't. The images are usable without cropping, but you can see a noticeable difference in IQ between ISO 200 and ISO 800-1000 with this lens.

If you like clarity and low noise, the camera performs much better at ISO 200 than ISO 800 when you lose the light, so the value of the PRO lens depends on if you find yourself in need of F/2.8 to keep the ISO down if the IQ difference matters to you.

Subject separation is also much better at f/2.8 than it is at f/5.6. But in landscapes you might be stopped down with a slow shutter speed anyway to get as much as you can in focus rather than isolate the subject.

Blanket statement - the PRO version is much better. It is much sharper at all focal lengths and apertures. Depending on what you are trying to do it may not matter. Enough to spend the money for it? Subjective. I'm glad I did and I paid 50% more for it than the next most expensive lens I own. I wouldn't take $1000 for it - a $300 profit. Its that good.

Another possibility is the 50-200 Four Thirds and the 1.4XTC with an adaptor. It is not as sharp as the 40-150PRO, but it is sharper than the 40-150 and it renders beautifully, even better than the 40-150PRO, but it focuses slow and is big and heavy. When I look at the images I made with it I can't give it up. MSRP $1,200, can be bought for $250, TC $150 = 560mm. Adaptor $100?