FWIW, I went from a 2.5" SLP cat back to a 3" cat back and lost 20-25 hp (on the dyno). Track times dropped from 13.0 to 13.3-13.4. Also upgraded to a Gen V while having the 3" on. Same track times (even going to a 3.0 pulley) (was on a 2.8 when I went 13.0X on a Gen 3 blower).

I put a 2.5" cat back back on but haven't been to the dyno or the track yet. Now I'm on a 3.0, gen v and 2.5' cat back.