The thing with just the one pipe and then a PHONY pipe, doesn't make any sense at all...I think it would be pretty obvious, pretty quickly, from residue and discoloration which is the functioning pipe and which isn't....That would be an improvement over the stock system??
As far as after-market exhaust, you've got 4-5 choices tops....Borla, Stillen, B&B, HKS, GReddy...Lipps, if I missed any, please fill in...
Replacing the stock is a tough call...Stock sounds really good, and I've heard both Borla and Stillen and they both sound deeper, fuller, but not by much...As far as extra HP...Again, unless you're racing...you'll never tell the difference...Is it worth $850-$1200 clams??? [dunno]
'03 G35 6MT Diamond Graph/Graph/Aero/Prem/Alum. Ped./Z Intake/Z Pop-Charger