Joe - I have the IS version and can recommend it. Yes, you retain AF with a 1.4 convertor, however, AF performance is slower on ALL Canon bodies with Canon lenses when using a convertor - Canon actually pass this off as a feature - something to do with "ensuring precision of focus". When I use the 1.4, I shoot in MF mode anyway.
The good news is that the 300 f4 has to have one of the most delightful focusing mechanisms I've ever used - and I've used a good range of Canon's top manual focus lenses. If all lenses were like this, you'd wonder why they bothered inventing AF in the first place.
Go for it - coupled with the convertors, its a very versatile package at a very managable size and weight ... and in case your interest ever strays from aircraft, the macro capabilities with the 2x convertor attached are nothing less than phenomenal .
Oh, yes, Mr Canon, if you're reading this, cheques or cash equally welcome
Colin K. Work, Pixstel