2002 bugeye wrx turbo with PPP.

Im slowly upgrading my car piece by piece, got the walbro on, got a straight through up pipe on with EGT bypass etc. Exhaust system has got a second de cat pipe on already, so i am wanting to maybe change the downpipe. So im looking at either a 200 cell sports cat or a straight through downpipe.

Will i see a spool up or performance gain with either downpipe ? Also if i fit a de cat pipe, how do i get around the engine light coming on ? I dont mind re fitting my standard downpipe once a year at MOT time.