Tim,
I'm having the manifold and downpipes coated inside and out.
The coating they recommend is a flat black satin finish, possibly as the coloured ones they can supply don't have the same heat retention qualities. The black satin finish is rated at 1000degC or 1832deg F
I won't be getting the turbos coated now as they cant do them in situ as they need to cure them to 400degC. The turbo specialist I spoke to told me not to disasemble them as they were in good condition and taking them apart may dislodge the back plate, I believe he said, which could mean a rebuild may be necessary.
Looks like a turbo beanie is what i'll go for.
Thanks for all the input.