I have logged both painted and unpainted intercooler air intake temps and the difference is not noticeable. So many other factors come into play that the insulation of paint is not relevant. Plus the 3% difference in temps from the two test could be wrong. The intercooler may have been heat soaked due to multiple tests and due to it being later in the day and increased ambient temps.
My buddy and I have almost the same setup, same turbo, same manifold, same intercooler and almost the intercooler piping. The difference between our setups is his is a det in a b14 running aem and mine is a vet in a b13 running nismotronic. We both used the same thermocouple to calibrate our intake air temp sensors. Now I have my hot side piping and intercooler painted black and his is not. We have compared intake air temps and they are almost the same on the same boost levels, his tends to run a bit higher under load.
The real change comes with boost levels, he is now running 18-19psi and I'm running 10psi. His intake air temps are much higher then mine now as axpected. For those of you thinking painting your intercooler will cost you hp I would say worry less about the intercooler color and more on system efficiency.