I just received an APEXi N1 EVO-R, advertised to have 2.5" (63.5mm) piping. As I was kind of expecting the 63.5mm is outer pipe diameter and inner pipe diameter is only 60mm. However, inner diameter at the exhaust flange where it meets up with the cat flange is only 57mm. This perfectly matches the stock cat flange which is also 57mm. But I was planning to get a header and HFC also with 63.5mm piping later on (like the Berk or EVS tuning 63.5mm HFC).

Isn't my 63.5mm exhaust supposed to have a larger flange diameter than 57mm? Are all 63.5mm exhausts like this? Are the 63.5mm HFC's also only 57mm inner diameter at the cat-to-exhaust flange?
Will I lose all potential HP gains if I mate a 63.5mm HFC up to a 57mm inner diameter exhaust?