850 sounds about right for an engine that has been running for awhile

I have not measured more than 1280 F
at short bursts of full throttle
at the hottest cylinder
on a 1995 360,
but I would guess in the worst case
it would approach 1500 F on the inside of the manifolds
and perhaps 1400 on the outside

The black magnetite coating
(like on impact sockets)
that people are trying to make on the outside
using the oil coating will break down at those 1200 to 1400 temperatures

$150 for a thermal barrier inside and another anti-corrosion coating outside sounds like a great bargain to me for a pair of manifolds