I have not heard any HFC's in person so cannot comment. I do know the effect of a test pipe though and if HFC's are similar, it will make even a quiet cat-back very loud with more drone. The Borla does have mild drone at cruising speed--sounds like a mid-pitch hum.

Personally, if Cobb ever does release accessport (with cat insufficiency code disabled/mitigated), I am going to go with the Berk test pipes.