Oh no! Do not put the extenders back in the closet! They work VERY well on the 300 F2.8 - they are just not as good as a longer "Big White" if you need more reach - but they will certainly do a good job most of the time.
Well, they slow down the AF considerably, on my 1DM4, is my experience. Superbikes are fast. And I have had good luck with simply cropping. I know it is not the same "look" and "pop", but it's better to have a shot in focus, to crop, than have a nice tight shot out of focus. LOL