right i have a 54 plate wrx which currently has a sports cat downpipe and a centre decat the piping is all 3" and has currently been remapped to 275bhp but im thinking of quieting it down as its quite boomy on the motorway by fitting a 2.5" diameter magnex backbox, will this affect the power output? bearing in mind im about to fit a vf34 turbo with bigger injectors so power hopefully going up to 320bhp will my power be affected then?

 

I wouldn't of said so, will be mapped anyway so will run to the best of whats on there.