I believe any lens with F/2.8 is compatible with 2x extender Version II to give full AF (A tad slower than if extender not used).
1.4 x gives AF on some EOS bodies at F/5.6 this is lens dependent also.
400mm F/5.6 L gives you AF with 1.4x if it is used on 1D, and manual focus if used with other bodies including 30D and 5D.
2x is only operable on manual, no matter what combination, when used with this lens.
You don't need to tap the pins as the pin coupling for electrical confirmation of focal length should function as usual. Taping is needed when you use the extender with a manual lens on adapter such as FD. In that case the chips in body gets confused of the lens' focal length, thus giving you error I believer 01.
The reason to use a prime lens is the extra ordinary sharpness it offers. Do you really want to decrease it by extender? Also don't forget the 2 stop drop in x2 that can turn a lens into very handicap if light not friendly, in addition to difficulty focusing manually due to very dark view finder. I wish the new DSLRs could incorporate some sort of prism split etc for old fashion manual focus confirmation or at least one such changeable optional viewfinder screen.
Hope I have not confused you.
