Firstly a decat will need a remap and if you are worried about noise a decat will p11s off your other partners as its loud.
Go for a scoobyworld afterburner vortex, i had 1 before my decat.
I went from a standard sti8 265 bhp to 343bhp and 332ftlb with a decat fuel pump custom zorst and a remap by prosport.