That, and we don’t know the long-term effects. A 10-15% increase doesn’t justify voiding the warranty, and their “solution” might just divert more heat to the case. Even if the CPU is at 100 degrees Celsius the case is fine with the stock setup. And a 10-15% boost still doesn’t put the Air in MacBook Pro territory.
And, yes, I get that I did say 10-15% is nothing to sneeze at. If you get the i7 you basically get the performance boost of the rigged solutions for the cooling system. But you void the warranty and make it ineligible for AppleCare or a third party warranty. I wouldn’t go through all the trouble and risk of a homemade solution.