I dont know how much it affects performance, but I do know that Perrin Motorsports makes flat black FMIC kits for the WRX and WRX STI. I helped my buddy install their kit on his 2010 STI and it seems like a pretty durable paint, wheather it be powdercoated or just painted, but the intercooler and piping is flat black with black silicone connecting hoses... So for a big name brand company to produce a FMIC kit for mainstream performance vehicles of that manor, would make me think that it DOESNT affect performance to the point that street vehicles will ever feel the performance difference, he is running 23psi, now yes I agree that without running a non-painted core and logging intake temps on both and comparing the two, it would be hard to say if there is a signifigant difference. I have seen a few people paint the end-tanks, that seems acceptable, and the IC piping, but to my way of thinking, it WOULD affect temps. I remember reading on here that someone had done a comparison of a painted and non-painted core and logged the temps and all, but to be quite honest, I dont remember the outcome of it. I would say not to do it, but thats just my 2 cents for what its worth.