People use the hpc coatings for pistons...so they must hold up pretty good...i know they do alot of race cars and drag cars cause theres genereally a pantec parked in their lot every month or so.
If your keeping the heat in...i would think since the flow is going towards the exhaust and outwards then it wouldnt sorta reverb back onto the head and block...but your engine is constantly being cooled anyway...if anything it would simply be decreasing the cooling job of the coolant making its job a little harder.
Every jap race car i see has their manifold completely wrapped. Good or bad thing? Gotta find a middle ground for everything, less heat radiation and improved heat energy flow or excess heat and wear on engine and attached ancillary items. Overall prob doesnt make a massive difference. Only massive difference is ofcourse decreasing the under bonnet temp as stated already. But for wear and performance issues prob not so much.