A 911 fan just delivers more cooling at lower rpms than a stock fan, but there reaches a limit where you just can't push any more air over the cylinders. The same net effect can be reached by changing your pulley ratio and keeping your stock cooling setup. On the other hand, by putting on aluminum cylinders, you can dissipate more heat, which makes better use of the limited airflow that can be pushed past the cylinders.
Charles Navarro
LN Engineering
http://www.LNengineering.com
Aircooled Precision Performance