Fasteners, unless they're specifically designed to take a permanent "stretch", are seldom torqued to the yield point. Exhaust manifold fasteners are not the "stretch to yield" type. In addition, most automotive fasteners are not of this type with the exception of the connecting rod and flywheel bolts on 911s and cylinder head fasteners on other cars.
I agree that stainless steel would be an ideal material for the exhaust studs as corrosion is an issue with anthing on the underside of the engine. If bolts are used in this application, not matter the material, the threads in the cylinder head (aluminum) will eventually fatigue.
Bolts, studs and nuts all work on the same principle of the inclined ramp.
Sherwood Lee
http://members.rennlist.org/911pcars