I know that this subject has been up before, but I can't make my search function working.

Could anybody tell me if there would be any gain in changing my up-pipe in a 99 GT with turbo back de-cat system and K&N induction?? I'm asking because i'm changing my TD04 with a TD05-16g in a couple of weeks and if would be a good time to change the up-pibe as well. I will be staying on the OE headers.

Thanks for your advice

Rasmus