(Mustang dyno, 178 pulley, Needswings manifolds, EC tune) Getting rid of the 4 cats, the rest of the exhaust stock, was worth 15hp on my car. Open downpipes gave me 11hp more. To be clear as possible, dyno verified, 26 hp with open downpipes and stock manifolds, resonator and muffler. Afterwards I installed long tube headers. Open headers (no exhaust) 7 hp over open downpipes. Connected stock resonator and muffler, 4 hp over open downpipes. This was all with the same tune. Results could vary depending on your particular tune and unknowns. I found and replaced a leaking intercooler afterwards. I have no idea if it affected the results or not.

Les