I still don't understand what you mean. AVCHD is still long-GOP H.264.
if you compress h264 poorly it looks like crap regardless of bitrate.
the 5D doesnt compress it well despite the high bitrate. Lots of bad food is still bad food... just lots of it.
a dedicated AVCHD camera compressing h264 with a well designed compression circuit will look better even at a lower bit rate.