How much power is this likely to add?

ta.

 

About 3 bhp on its own IMHO. Don't look for huge gains without the rest of the system being done.

F

 

Certainly not too much... the noise is "gentle".
I have compared to STIs, both with sport cat, one with Prodrive backbox and one with a Magnex, the Prodrive dealer said about this one : "great sound and feel to be more efficient ! "