I don't know what's on C-band, but here's a
couple of lists from Global-CM.
There are three or four members running some C-band on 4' dishes.
They can give you a list of the hotter birds.
edit: here's one of our resident
4-foot-for-C-band proponents.
Some users with 6' dishes seem to do a lot better.
It'd apprently take 8' or so to run a 4DTV receiver on C-band and get the remainder of services.
Think it's more of a beamwidth problem, and adjacent birds interfering with the one you are pointed at.
And of course, satellite transponders with FEC like 7/8 don't have much margin for noise (interference), as would 3/4 or 1/2.
The subject has been discussed before, so you might find some more info with a search.