I have put the Stainless Specialties mufflers in a SPF like Deans. The exhaust seems to have a lower rumbling sound to it now. It sounds good and is comfortable in the car. It is the single biggest performance mod made to this car to the point it gave traction problems. The original SPF exhaust was crimped across the inside tube so there was a 3/4 x 1 5/8" slot left open. Horrendously restrictive.
I have put several sets of Classic Chambered in to Cobra side pipes. I have heard the 2.5" with the louvres facing towards the motors one of them being Karls above. I can still easily chat with my daughter while we ride around in these cars. They seem to make good power. There is no comparison to the 1.75 or 1.625" ID's these cars had. The customer of one set says he wants to reverse the mufflers as the flapping noise is driving him crazy. On the other hand they sold me who is a street car undercar exhaust fan on side pipes, to the point where I carry these mufflers in stock.
The 3" Classic Chambered I have also used with the louvres facing forward, we haven't heard those run yet. Classic Chambered recommends the 2.5" good for up to 450 HP and they become restrictive after that. I am using really nice transition cones at the front of the mufflers so it doesn't create turbulence. The performance specs are on the site and some sound clips of Cobras too
http://www.classicchambered.com