You'll probably be fine. Although it's true MacBook Pros use the case for heat dissipation (that's why they get so hot) your mod shouldn't be any worse than those form-fitting Spek/etc cases.
Also.. looks pretty slick.
iMac (27-inch, Late 2009) ~2.8GHz, 8GB RAM
Mac Mini (Mid 2007) ~2.0GHz, 2GB RAM
MacBook (13-inch Mid 2007) ~2.16GHz, 2GB RAM
MacBook (13-inch, Late 2008) ~2.1GHz, 4GB RAM
iPhone 4 ~32GB / iPhone 3G ~8GB