Hi
Lowering cabin temperature has not been my experience after fitting SS Manifolds. They are fitted to improve performance, however, unless designed and fitted with a number of other changes/enhancements they could lower BHP. The standard manifolds are well suited to a standard engine.
The other reason to fit them of course is appearance. Look on:
www.etypepat.f-snet.com the click on the Lightweight Replica section for a good example. I had these made by Haywood and Scott and what you see is exactly how they came, fitted with no problems.
Best of luck
Pat