Well, I finally got the testpipe on today. It was a bitch to put on. Anyway, I started her up and the engine sounded like it was idling fine. I let her warmed up and the boost gauge was around 10 vac. during idling. Usually, it's around 15-18 vac. when I had the stock cat. I take that this is normal since the exhaust flow is much more free. I took her for a spin, the car seems to pull a bit faster, but I was only hitting 1-2 psi. Back then, I was hitting like 3-4 psi. Also, at WOT there is a 'hissing' sound coming from the engine bay. Back then, I could only use to hear a 'whoooosshhhh...' sound coming from the wastegate. Now there is this hissing sound. The car pulls fine and all. Cooling down, I checked all the signal hoses and all them are snug. Fuel pressure is at 55-60 psi.

you have a vacuum/boost leak somewhere.

your idle vacuum should still be the same w/ the testpipe.

that hissing sound is a leak somewhere

Are you supposed to used the stock gaskets with the testipe??? When I tried fitting it, it doesn't seem to fit properly. I'll see if I can tighten the bolts some more.

Get larger exhaust gaskets from an autoparts store. The stock ones are only 2 inch and will disturb exhaust flow in a larger pipe. If you have a 2.5 inch testpipe get 2.5 inch gaskets. The metal crush type from felpro are what I am using. $5 each I believe.

I also agree you disturbed something under the hood to get a leak. I heard a leak from inside my car and it turned out to be the boost gage connection. I personally replace almost every plastic vacuum line with silicone and have zero problems. Most problems occur where all the air lines connect around the throttle body. Also check your BOV for a tight connection. If you still have a PCV valve connected to your throttle body/crank case it might not be sealing properly anymore.

Ya, I thought it was the testpipe, so I took it to my friend that works at a local service place. I had him impact wrench the bolts and no leaks from there. At least, not yet. The 'hissing' sound I heard was actually inside the cockpit, it was a connection on the boost gauge. Anyways, I'm gonna see to fixing it tonight, we'll see anymore leaks though. Thanks guys.

Well, how did it go?