I think the built in TC flexibility is just as important (fast) as pre-capture or any af difference. Pulling a lens off to add a TC is a pain and can't be done when you need it right now, you'll miss moments. It almost turns a prime into a zoom. The TC is not something I would compromise on for high end lenses.
Pre-Capture doesn't do anything a large buffer cannot if you don't mind scrolling through the shots, which is sort of like fast forwarding video. It's more a convenience, a nice one, but you'll still get the shot with Nikon's buffer. It's essentially unlimited with the right card and HE*/HE in use. The A-1ii is using its equivalent after all to shoot 30FPS instead of lossless. I know it's a bit strange holding the shutter down for a minute or more, but it's no different than just recording video clips. There's no shutter to wear out.
If I were you I'd do what Steve has and buy the A-1ii and use it with the 300GM and TC's and keep the Nikon gear. You'll have a Z9ii in a year and the 10 FPS and pre-capture difference will be gone, likely the AF difference as well.
That way you'll constantly have the best kit on the shelf any given year. If I had the money that's exactly what would be on my shelf.
Right now the best lenses on the market are those Nikon TC's and Sony's 300GM, just have the bodies to run both.
All you need is to add the A-1ii. Heck sell a Z9 and have one of each.
People always bring up the "just shoot with an infinite buffer" argument, and I don't find it valid. I want less garbage images to sort through, not more. Also even if unlimited buffer, my Z9 cannot seem to get good frames going from static perch to in the air. Not sure if it's a function of the AF, the "only" 20 FPS, or what.
Having tested the R5 II, I was able to use that system to get every shot I wanted with ease.
There's no point to me in keeping Nikon gear for a "what-if" situation in a year or two. Camera gear prices only decrease, and I'm not interested in being a multi system shooter.
If Nikon leapfrogs and brings back something interesting (hello Z9 II + 300TC???), I can always swap back easily.
This is quite a statement, I would probably highlight YOU BELIEVE they do not provide any value TO YOU as the the concept of outperformance is very subjective.
I think you're taking it out of context and too broadly. I'm saying for anyone that can afford the 300GM + TC's, they outperform the 400 4.5 and 600PF, meaning anyone in that situation would not have a need for them.
I've yet to see anyone suggest otherwise.
Outperforms in terms of AF? Outperforms when used naked? I have no experience with those Nikon lenses, but optically I would expect the 600PF to be superior to the 300GM with TC2.
Outperforms in terms of image quality and flexibility.
I think if the 600PF wasn't a PF lens, I would tend to agree. But in my experience with PF vs "normal", the PF lenses have a lot more "restrictions" when using them. Their images fall apart more quickly under inclement weather (haze, atmospheric conditions) or when shooting at further distances.
I also just find the IQ and bokeh of the 300GM + TC's to be much better than the PF lenses.
When I compare the 300GM + 2x to a 600TC, they look to be in the same caliber. When I compare the 600PF to the 600TC, I don't feel the same way.
As always, YMMV.