Don't sweat the fact that most of it isn't exposed to the air either. The more I read about intercoolers is that they are really heat sinks more than radiators. You aren't in boost for long periods of time but rather short bursts followed by periods of non boost. During boost the intercooler is really just absorbing heat from the intake charge(for the most part). It will dissipate that heat to the ambient air when you aren't in boost. Road race car maybe a different story, but street/strip, a 5 sec. pull here, a 15 sec. pull there and it's not all that important to have direct airflow.
I don't think I'd cut my bumper for 500whp. I'd just run a bigger heatsink(intercooler) to absorb the heat during the 9-11 sec. pull.