As kabab said, on the R33 GTS-t, you will maybe gain 1-2 rwkw by doing the cat-back section. This is because the restriction is found in the dump pipe, not the cat-back section. After going the dump, front pipe and 3" cat, I noticed a HUGE increase in power, but didn't notice any power increase at all when I bolted the cat-back section on.
Air filter was probably good for another 1-2rwkw...
So at that stage, I managed a 14.3 with a STOCK CAR (plus 4-6rwkw in mods)... Hardly earth shattering mods.