Feels like my 2004 GTO has power lag or something?

I have a 2004 Pontiac GTO.Has 30,000 miles, 6 speed transmission. When I first got it, this car was really fast. Now I have put SLP underdrive pulleys, 3:90 rear end gears, SCT Flash tuner, x-pipe with straight pipes and stock cats. Had a cold air intake but took it off. My buddy owned this car before me for about a month, and he drove it a couple of weeks ago and said he was shocked of how much slower it is now compared to when he had it. When he had it, all that was done to it was a dyno tune. The best way I can describe what is wrong with it is it seems like it has power lag. If that makes sense? It seems like there is a turbo on my car with major boost lag, but there is no turbo. When you stomp on it, it takes like 3 seconds for the power to kick in. The RPM's are going up, but no power. And when you downshift, it seems like you can feel the power in the throttle, but something is just completely holding it back and not letting loose. The only thing wrong with my car is an exhaust leak. But there is no way that is hurting my car that bad. Please tell me you know what is wrong???