These cars respond pretty well to headers but Spec cars have to run factory manifolds. I'm guessing that the stock manifolds are the most restrictive part of the exhaust and you aren't going to see much difference by changing things downstream.
I remember someone doing a comparison on a 944 Cup car with headers and finding very little difference by changing from stock exhaust to aftermarket muffler to open pipe. IIRC, they were all within a couple of HP.
Typically the more open the system, you gain a couple of HP at the expense of a couple of TQ for no real net gain.
