Specialty Z recommended against coating their inconel manifolds, but it is not a problem to do it to a cast iron manifold. My manifolds have been coated for 36k miles, and have endured several track events without issue.
I wouldn't get too carried away with blanketing the manifold with insulating material though. Too much insulation can cause hot shut down heat soak issues for the turbo. Just ask CaliguyPurdue his thoughts on that :)