I put Classic Chambered Exhaust cobra packs on my car last winter. Much quieter, and a softer sound than the stock FFR mufflers they replaced. I don't think they gained me any power, but I don't think I lost any, either. I didn't do a before and after dyno run. But the Butt-Dyno doesn't lie - much.
As for heat, that's not really a function of the muffler. I sprayed mine with a ceramic heat barrier that seems to work well. The pipes had previosly been ceramic coated silver from Jet Hot. after welding the old tips to the new mufflers, I sprayed them with a black ceramic coating from Alvin. After a 20 minute track session, I checked the inside and outside temp at the turn out - the only place I could easily get to both sides. The differance was 50*.