Think about it..... Ford has gone to all the time and expense of designing, manufacturing and installing that snorkel to make sure the engine gets nice cool air from the fender well instead of that ugly, power robbing hot air from under the hood... Why do you suppose they'd do that when we all know they'd sell their mother for 2 cents?
Hot intake air increases the potental for detonation (detstroys engines) and robs horsepower. I think it's something like 1 hp for every 10 degrees.
Kenne Bell feels so strongly about it that they will void the warranty on their blower if you draw air in from under the hood.
I'd leave it on until you can get a quality CAI that goes into the fenderwell like the "Tunable Induction"
www.demolet.com or the Kenne Bell CAI
www.kennebell.net