I have no personal experience with a professional coated manifold but i would think coating the inside would be a bad idea.
Yeah turbo spinning 100k rpm and some chunks of coating hitting it sounds bad.
I have personal experience w/diy stuff from Techline... Think about it. Where do you want the heat? Not in the metal. As you raise the temperature of the metal the weaker/softer it becomes.
So yeah, let's coat the outside and hold the heat in the metal... wrong idea folks...
I coated the piston crowns, valves, and exhaust ports of my vitara slugged a6 w/cbc1. My engine has probably 4/5000 miles and 20+ 12second passes on it. And guess what, I've used the same turbine wheel the whole time.
Prep is key to make the stuff stick. When the instructions say to blast the part w/120grit aluminum oxide before spraying. What is one to do? pay attention... The chemical engineers didn't write that shit down just to nag you with something that's a pita.
So yeah if you're gonna coat the outside for looks, go ahead and coat the inside for performance too...