The thermal barrier will cut your underhood temperatures and that is most certainly worth more power than whatever loss of inside diameter the coating might result in,the coating is not that thick,will also keep everything else in the enginecompartment alitle happier by keeping the heat where it belongs. If the difference is measurable? dont know,never measured,but i know for a fact that many years ago i swaped from just painted headers to a set of coated headers with the thermal barrier and the difference in underhood heat radiation was obvious.
And you want that barrier inside the tubing not on the inside considering how some headertubing seems to harden over the years due to the heatcykles..
