There is definitely a big difference between AF systems of cameras.
If you get to know how to use the 7D's AF system - it's much more capable than the XXD line (faster, more in challenging listing, more AF points). The XXXD and XXXXD lines are even more limited in terms of AF capability.
Also, there is a big difference between 70-300mm nonL and 70-300mm L in terms of sharpness, contrast, focus speed / consistency AND IS. The L is noticeably superior in every regard. Also, though there have been a few 70-300mm L dud lens copies (eg as tested by SLRgear) - but in general, compared to the 100-400mm L, the 70-300mm L is better in AF speed and also has the edge in sharpness / contrast.
I'd definitely love to have a 600mm L II and a 2x teleconv with a camera that can do f/8 AF in some situations.... but hey, then it's not going to be an 'affordable' price! However having said that, for some reason I seem to get closer to some birds than a lot of people say... eg I even have cropped with small birds at times. Ok, not every bird!!
All the best.