Will exhaust wrapping reduce under bonnet temperatures and does wrapping make the stainless headers split(Iread that somewhere)
Exhaust wrapping will have a minimal effect on the engine temperature but does reduce the temp of the headers so lessens the chance/severity of burns.
To improve the gas flow from the exhaust you will need a freer flowing system such as Krazy Horse, MMC AR, Garage 56 etc or a "baffleectomey".
I've not heard of any M3W headers splitting after being wrapped.