I used my R8 for a lot of things, but the FGT will take you further. R8 can't compete with the cars in the later races.
Are we talking about the Road version R8 or the R8 race car?
There's also 2 R8 race cars aswell:
R8 LMS (a GT3 class car)
R8 '05 (Le mans race car -but don't know what class it's built for, but it's alot lighter and more powerful than the GT3 version).