Just like moving from a 60lb to a 70lb bow. However, I think I could argue the fact of a lighter for draw arrow (5gpi instead of 8-10gpi) on the 80lb, thus gain a bit more speed. You would still have a heavy enough arrow; Actually it would be heavy considering what one may have used in a 70lb rig. Just my $0.02
There are a few bow/arrow calculators on-line that would lay it all out. Its pretty slick and interesting to play with.
Biggest problem with an 80lb bow, most guys can't pull them...especially when cold.