Ive just taken off the stainless back box that came on my Williams when I bought it - and replaced it with a totally standard one.
To be totally honest, there seems to be no drop in performance whatsoever and the noise is about the same as when the stainless back box was on (still quite noisy).
Compared to my old Valver (which had a full system, de-cat and IK for breathing), there no roaring, but its pretty meaty for a standard system with an airbox and no de-cat.
My Willy is now mechanically as it would have been from the dealer.
So, dont bother getting a stainless back box unless you really want to get a full system. Just not worth it! The standard back box even looks nice with a fat tailpipe