i wouldn't be at all surprised if you need cats, and your egr valve's sticking BUT! check those hoses on each side of the down pipe ( the bent part right after the exhaust manifold ) and see if those are there/intact. but Nitrous oxides is almost always a cat or egr problem, and someone mentioned up there timing, that can be true too, if the timing is too advanced your cylinder head temp will be high resulting in more nox but first see if those hoses are there and start shopping for cats......on vg30dett cats rob about 40hp so a lot of TT guys go straight pipe, ask one for their cats ^_^ friggin car's got 4 of them bitches