My mate has the 944 s2 with virtually the same 3 ltr 16v engine in it as the 968. (968 got vvt).

I built him lovely 2 box (straight through) all stainless system. I used 2.5"

It sounds spot on and i tracked down the correct oval tail pipe so it looks like a stock porsche bit of kit. Materials for whole job including two stainless silencers was under £150.