you won't get BHP gain from changing just the rear box unless you buy something like an Eisenmann which if your lucky will give you a few bhp anyway.
If you don't want too much loudness, go for a custom exhaust ie Powerflow and get their quiet rear box. Shouldn't be drony and will give a little more sound compared to the OEM box.
Other thing to consider is budget?
Custom route will be cheapest, and buying brand names like Eisenmann or Supersprint will be expensive! Eisenmann do two versions of their exhausts road and race, the latter being the loudest. You will want the road version if you go for it.