I have used the 300 f4 with and without the 1.4 TC, and been very pleased with the sharpness and detail. I also have the 150-600mm if I need to shoot distant subjects. If I put the 150-600mm on a tripod and keep the f stop around f8 I'm able to get sharp images.
A few months ago I was seriously thinking about getting the Big White 150-400mm +1.4 TC. I liked everything about that lens except the price. It just seemed way too expensive. I then began to look at what else I could buy for the same $7500 price, and found lots of really nice Full Frame kits for Much less money.
At about the same time the OM1.2 AF was not "sticking" to the fast moving birds. The OM1.2 acquired focus ok, but wouldn't hold the focus as I panned the image. I realized that several camera dealers had a 30 day return feature, so I decided to find if the FF cameras AF was any better. The Nikon was Much better!
The Nikon Z8 + Z100-400mm was selling for about $1,000 less than Just the OM 150-400mm lens. Once I tried the Z8 I have not used my OM gear, which I still have.
So I guess the Om 150-400mm is responsible for me switching to Nikon. Yea, I know that the reach of the 150- 400mm is Much greater, but for the cost of adding a TC and the ability to crop Nikon's 45MP sensor, I'm good. I still have the OM 300 f4 + 1.4 TC +150-600mm If I have a need to for distant subjects. :>)
I went exactly the other way around: I sold my 300mm f/4.0 IS Pro, my 12-40mm f/2.8 Pro and my 12-100mm f/4.0 IS Pro. I brought the 12-100mm with the Big White on a Safari to Murchison Falls National Park in a two body set-up, I got very nice shots but did bump in to the gap between the 100mm and the 210mm of the Big White with the MC14 on it (I have that thing pretty much glued on it, only nuisance is that one of the screws of that thing get loose after a few days and needs tightening). I got myself a used Sony A7R IV with the 24-70mm f/2.8 GM II, so I made the move to a 3 body set-up, using a harnas. And kept the 150-400mm. I will confess: I did look in to Sony, Sigma and Tamron alternatives for Big White. I have come to the conclusion that, at this time, even when I'm taking the massive crop option that I have with my 60MP Sony A7R IV in to account, I can't find an alternative that is just as portable as my Big White AND has such an incredible short Minimum Focus Distance. Yes, the sum I paid for this lens remains absurd. But so is my satisfaction that I get using this massively versatile lens. (May I put the internal focussing under attention? That contributes soo much to hand-held shooting at absurd focal lengths, such as 700mm MFT, 1400mm FF equivalent, even when I recalculate the crop option of my Sony, it is still close to 1200mm Sony A7R IV equivalent).
And yet, I'm also one of those suckers that stepped in to that Full Frame Fairy Tale. Now that I own one (and I'm inclined to think that the Sony A7R IV is one of the high end Full Frame System Cameras out there) I can say that it does out-perform the OM-1.2 on noise and resolution, but at the same time I must add that, in good light conditions, both areas of out-performance are pretty much non-visible when printed on a maximum size of A3+ (13x19 inches).
The OM-1.2 however outperforms the Sony with things like ProCapture in RAW, in-body focus stacking, Sync IS (IBIS and OIS, for example with Big White).
In conclusion: for me the best set-up is the set-up that I currently have: 1 FF body aimed to do the "standard" shooting, 2 MFT bodies for the tele range and super tele range. Thanks to the minimum focus distance, I can even shoot macro type shots using focus stacking with either tele zoom lens that I have in my kit. I won't part from my Big White (I call it "Alma", after my late mother and father) as long as there is no portable Full Frame alternative ticking all the boxes that Big White ticks.