One big problem that nobody mentions when replacing the factory seats is removal of the eddy currents that help evacuate hot gases from the exhaust pipe and keep them away from the undercarriage. The factory seats and other components can deal with the increased heat obviously but the accumulation of thermal energy in the exhaust eventually works its way back due to conduction through the muffler, frame, cats, and eventually right into the exhaust manifold. Once the exhaust manifold is hotter than normal, you're in the head.
The differences in coefficients of thermal expansion of the aluminum head and the cast block are normally within the allowable limits of the elasticity of the head bolts but, with the increased heat from what I described above, its very likely you'll end up distorting and eventually blowing your head gasket. I see it all the time.