Will beat me to the punch about stacking 1.4 TCs on the 300/4 IS. I've only used mine with a Tamron 1.4 for 5.6 and it gives excellent IQ. I'll mount stacked TCs on the lens tomorrow and give it a try at 600mm/f.8 and see how it does.
I had decent results with 300/4 (NON IS) and 2x (MK1) on my 1D.
I agree distance could be the problem, here some shots taken at about 450 meters from moving subjects. Autofocus is slow, yes but accurate, no problem tracking an aircraft in landing even if I have not 100% of keepers (while with 1,4x I have no problem).My main issue is color rendition.
In these says I just considering to purchase the 300/2.8 IS just to have nicer results with the 2x (it is supposed to be a good performer combo) when needed (500mm is out of my possibilities), but at the moment I'll stay with f4.
