OK assuming a stock UK newage WRX, what is the gain from an up-pipe decat with no other mods?

Simon

 

slightly faster turbo spool up i suppose. much more to be gained with decating the down pipe and or center pipe.

 

Ok I thought the up-pipe cat was more restrictive than that - its a hypothetical question anyway as mine is cat less, but as I'm fitting the later spec turbo I was trying to guestimate what I would gain over that spec without the cat it has....!

Simon