If you see a torque dip after installing HFCs or going catless the likely culprit is the tune or lack of tune. I added a Fabspeed xpipe w/HFCs to my bone stock M3 and I had a tq dip at around mid rpms. When I added the Powerchip tune the tq dip was removed.
Based on my experience adding a catback exhaust, after market filter, or aftermarket pulley provides zero power gains
I’ve been diligent with before and after dyno’s for each mod I've added and the RDSport Exhaust, Akra exhaust, AA pulley, and MS filter gave me no hp or tq benefit.
Other than a supercharger, the only bolt-on part that seems to guarantee power is cat removal.