Which extender version for 300 2.8 IS v1?

I picked up a 300 2.8 IS I from a local guy who had used the lens twice in the last 9 years since he purchased it new & the lens looks brand new with all accessories, etc. I want to add a 1.4x extender and I'm trying to decide whether to go with the 1.4X II or 1.4x III. Is there any significant improvement in performance between the two on this lens? If you've used both with this lens, I would especially be interested in your feedback.

On the version 1 IS lens, you won't get any of the AF benefits that the MkIII T-Con offers.

Were I you, I'd go for a MkI or MkII 1.4x and save a lot of $.

The difference between the 1 and 2 is weather sealing and optical coatings.

I am not quite so certain. I had the Canon Mk2 1.4 and 2x extenders which I sold on after getting the Mk3 versions. At that time I had the Canon 300 F2.8 L IS Mk1 (still got it) and the Canon 600 F4 L IS Mk1 and I found a marked improvement in IQ and AF with the Mk3 2x extender. With the 1.4 Mk3 IQ was the same but I find the AF to be a touch better in my experience. Cameras used were the 1D4, 1DX and 7D2.

The 1.4 Mk2 and Mk3 are very close IMO but I slightly prefer the Mk3 - could be sample variation(?) the difference is very small. If you decide to get a 2x in the future then get the Mk3 - it works very nicely with this lens - much better than the Mk2.

There does not appear to be a real convincing consensus of IQ difference with the 1.4x 2 vs. 3. Some detailed measure-baiting has revealed one is sharper in the center, the other sharper at the edges,. I can't recall which.
I am confident that Canon DID make IQ improvements with both of the MkIII T-Cons, but with the 1.4X, I could not tell the difference in use. The 2X seems more improved.

IMHO the difference in IQ is such that it is a subjective conclusion as to whether it is enough to warrant the extra cost. My opinion is that it is not. My opinion is that the reason to go with a MkIII T-Con is to take full advantage of the latest in Canon;'s AF tech.

As for AF, perception aside, there is virtually no way that the MkIII T-Con's can improve AF performance unless it is paired with the correct equipment that will allow the superior AF to function. Sadly that list of specifically equipped lenses and camera bodies is very limited and does not include the older Gen 1 IS super telephoto lenses. We can not expect a lens designed and launched in the 1990s to be able to work with the latest Canon AF system advances of just the past few years. Therefore, if you are missing any one part of the three ingredients needed to achieve improved AF function, (T-Con MkIII + latest camera bodies + latest lens) then you can not get the new fangled AF. It's easy to tell, count the active AF points @ f/8. (f/5.6 lens with 1.4X, or f/4 lens with 2X)
If you get all of the AF points, you are in AF nirvana. If not, then you have old school T-con AF with the brakes applied.

Back up a few years, from the time Canon has had autofocus lenses and teleconverters, Canon has imparted an intentional decrease in AF speed from the moment a t-Con is mounted. Canon AF has preferred to forgo speed in the name of AF accuracy where T-Cons are involved. Similar to it's AF @ f/5.6 only bodies, Canon has preferred to remain conservative as to how much light it's bodies require to try to AF at all, and with T-Cons at what speeds it will try to achieve AF. toss on a 1.4x and drop your max aperture from f/4 to f/5.6 and the whole system has always automatically shifted down a gear. Precision over speed. Put on a 2X, it shifts down two more gears, and only allows the center AF point. (on older bodies that even allow AF @ F/8. Most bodies, no AF at all)

It was not until the 1DXII, and the very latest lens/T-Con combos that Canon has had AF quality taken to a high enough level to discontinue the "AF Handbrake"
Yes, some slowing can still occur, but this is due to actual lighting or contrast issues, not Canon dropping the AF speed at the mere presence of the t-Con.

When the MkIII T-Cons first came out, the reports on this forum and elsewhere of improved AF spurred me to do a lot of testing on my own, and frankly the results all flew in the face of any perceived (but never actually "measured") improvements in AF that people were casually reporting. Likewise I've read no measured reviews from trusted sources that have the ability and time to actually measure such things that state or even imply that one can garner the AF speed improvement without the entire food chain required to do so in place.

EG:
- Even with the correctly equipped lens, the new 500mm IS II and a T-Con MkIII there was no real AF improvement until I also had a 5D4 to pair it with.
- With the EF 500mm f/4L IS version 1 side by side with three bodies swapping MkII and MkIII T-Cons, no real perceivable difference in AF.

Get all three together, in my case 5D4, EF 500mm f/4L IS II, and 1.4x or 2x MkIII
Everything changes!
You get all AF points working @ f/8 and there is no immediately noticeable AF slow down. Canon's old "T-Con hand brake" is no longer there. In good lighting, AF appears to be as fast and confident as the naked lens.

get the latest unless you really believe it makes no difference and there's no way you will ever own equipment where there is a difference -- i.e, you can see into the future.

Good points and detailed explanation Jake.

I was just reporting my experiences with my (listed) gear. The AF improvement on the 1.4 was very small and may be subjective but it was just enough to make the switch worth it for me. Officially there is no AF improvement with the Mk3 extenders unless you have the right gear - which I haven't, yet I find the 1.4 Mk3 just a little better (the 2x Mk3 is very much better). Funnily enough a friend of mine (who now has my 600 F4 L IS) would not buy a Mk2 extender as he found them too much of a compromise - he now has both Mk3 extenders and is quite happy.I am not saying it is wrong to get the 1.4 Mk2, I just found the 1.4 Mk3 to be a touch better.

Incidentally you mention edge sharpness, apparently the 1.4 Mk2 is sharper at the edge of the image than the Mk3 - that's progress for you!

As I said just relating what I have found to be the case with my gear.

Thanks for the great discussion and feedback. I thought long and hard and was going to purchase the vII but decided to go ahead and plunk down the extra cash for a used vIII since I own the 70-200 2.8 IS II which does have the IQ to take advantage of the newer version along with my 5dIV. I picked it up through a local camera club for $340 in pristine condition, which seems like a fair price based on what I've seen them go for recently.

Sounds like a good choice. Particularly with the 5D4 already in your line up.

Either the 1.4X II or III... doesn't matter with that lens.

The II might even focus faster. Though I doubt it's enough that you'd ever be able to tell. (Both are "near instantaneous".)

The III was developed along with and optimized for the "II" version super teles.

Anyone finding differently might want to use Micro Focus Adjust with their lens and TC combo.