I get the buzz at around 4k when the car isn't fully warm and I'm accelerating hard. Anyone else have this?

 

yup i get that same buzz...i recently switched back to my stock exhaust system (sounds incredibly with my setup) and the buzzing was a lot less..but it comes and goes...somedays the buzzing is there for a while but sometimes its gone after 2-3 min...and yeah its only at higher rpms...you think by putting my stock cats back on im gonna notice a big loss in performance?

 

well if anything is going back to stock its gonna be the cats since thats whats causing the buzzzzzz