the big kahuna

I dyno my car the other day and made good RWHP to 5800 and then it falls off about 20 hp and runs out to 6800 doesn't seem like valve float it a 239/243 cam 403 LS 2 now I have a 3 1/2 " exhaust with a strait though truck muffler I have magna flow high floe spun cats the good ones I am taking a pressure reading between the header and the cat I got 2.8 psi to 5700 and then 3 psi past that I was thinking 3 psi is a lot I took off the muffler and now it 2.8 psi past 5900 so im wondering if these cats are doing it I was told they would rob about 10 RWHP but this problem is more then that instead of dropping 20 rwhp should be going up another 15 rwhp at least well i dont know if this 3 PSI in the exhaust is the cause???