you can get the Kenko and tape some pins to get f8 af, but it will be slow.
Actually, since I got the Kenko (Pro300 DG, there's a lot of other different versions, your experience may vary) 1.4x, I've figured out that you don't even need to tape pins.
If the lens takes canon extenders (ie like a 70-200L), then it has 10 pins.
If it has 10 pins, then the kenko converter will translate Fmm and f/ properly, ie 70-200 f/4 becomes 100-280 f/5.6.
If it has 7 pins (like my 70-300L), then the kenko is "invisible" to the camera, ie, the EXIF data still reads 300mm f/5.6, even though it's 420mm f/8 in reality.
So the camera "tries" to AF, but unless it's in very bright sunlight (even if it is), it hunts and fails a lot because the aperture is just too narrow (for my 7D at least).
But hey, seeing as I already had the 70-300L, adding an extra $100 wasn't bad for a Manual Focus 420mm f/8 IS lens on top.