You will definitely get better AF performance and equal IQ with a 1Ds MK II and cropping. However, f/8 performance with a extender wasn't bad, just not lightning fast.
You really do not need to ratchet focus from close up to 300 ft., use the AF limiter or pre focus. Then with a slightly different distance, focus is almost instant. I found the same thing happened when autofocusing at f/11 with a non reporting setup. Manually focus to be close, and a tap of the shutter snapped the lens into sharp focus where it would not focus at all if the focus was off by 100 ft.
Recently I tried a Kenko 1.4X TC on my 5D MK III with my 100-400mmL, AF surprised me with its speed, particularly if the change in distance was not great. I returned the Kenko due to issues with my 100L, and am waiting for the promised f/8 firmware update.