What I've learned, was that the efficiency is not affected as much as some would think. But some do paint them with special reflective coating type paints. I personally don't care for the paint, and would much rather have the cast aluminum look, but I can understand and respect wanting to tone down the fact that you're packing a huge intercooler for your street encounters. The painting practices that I saw were not to pile it on or make the coat very thick, but to give it a nice even coat, couple of passes so to speak. The other thing to remember is, Buick engineers decided to use painted Garrett intercoolers on the cars from the factory, and this paint was a special type of paint. What that "Special" paint actually is or was, I do not know. In all my years of playing with not only Turbo Buicks, but also other turbocharged and intercooled applications, I have never seen any published testing results of paint vs no paint on intercoolers. Guess someone needs to do some testing.
Hope some of this helps.
Patrick