in case u havent read my PM yet...
i had the same problem when i got my cat-back.
1- advance your timing 2 degrees (if it isn't already)
2- reset your ECU
that should help quite a bit..at least in my case it did.
you might also look into getting APEXi silencers. they let you tune your exhaust by adding discs to your tailpipe. sounds ghey but if you are after that lost backpressure. its a sure way to get it back...and you can fine tune it by adding/removing discs. check them out at www.apexi.com
hey man, APEXi is having a garage sale at their shop in Orange, March 8th and 9th. they might have them on sale. I'm gonna go to see the cars their gonna have on display too. (skylines, supras, etc.)