How much difference in hp would i get between going with a decat downpipe and sports cat downpipe?

The car is an 03 Sti with full vortex system including decat centre and angle exit rear box. The car will also have an aps cold air induction kit when it goes in for a remap.
I just wanna gauge whether its worth going for the full decat or getting a sports cat so i don't need to worry about emissions when it comes to MOT time.

Thanks for any help given

p.s i have done a search but can't really find a good answer.

 

I think Xtreme tested a sports cat DP vs a Decat, both supplied by H&S, on the same car and found a nominal difference (couple of horsepower). The real difference is in the price! I think the sports cat downpipe is best part of £500!

NS04

NS04