I think you'll be thrilled with that combination.
For me the jump from 7D to 5DII was a larger quality jump than any lens upgrade I've ever made. It's true that with proper light you'll never be able to tell the difference between crop and FF, but I often shoot in less than ideal conditions, and while the 5DII still isn't a "perfect" camera for me, my keeper rate went up substantially going from crop to FF.
This was also at a point where I had the 7D plus L lenses so the lenses were not the limiting factor. I had gone by the old adage of upgrading glass first, although honestly if I had to do it again, I don't think I'd do it that way. That was true in the film days, when glass was often the limiting factor, since you could also easily "upgrade" your film with a different stock. These days, your camera IS the film - and it's not like you can drop in an upgraded DPAF sensor into your old 20D. And with sensor improvements dramatically outpacing lens improvements I would say make sure you have a body that works for you and then find the best glass you can for it.