you should get a K&N, and remove the air silencer from the airbox, its on the fenderwell side, you'll have to dismantle most of the intake box parts, but its very easy, i think there is a writeup about it at
www.sn95.com in their tech section... the 4.5 and the SN95 have similar intake boxes i think...
that'll be a good place to pick up some free HP, i would say, with K&N and removed silencer, you'll see 10 rwhp.
if you have to take it to the dealer, they wouldnt notice that the silencer is missing cuz its on the inside of the fender, and the K&N panel is a stock replacement filter so it wont void any warranties.
FORD !!!!!!!!