In my opinion, you're better off saving the money for some bigger gain in power.
Also, I don't like the idea of my water pump and alternator turning SLOWER.
Hell, I installed a SMALLER (ie: overdrive) alternator pulley on my Trans Am to get better charging at idle.
2007 Grand Marquis LS: 5.4 swap, 9.9:1 compression ratio, Comp XE268s, NPI pistons, PI heads ported by me, Accufab upper plenum, 75mm DBW throttle body, Factory PI intake with Chinese spacers, CVPI dual exhaust, CVPI torque converter, CVPI air-box and zip tube, J-Mod, 3.55 gears, FMS Trac-Loc differential
17" Dorman CVPI steelies, BFG Comp T/A 255/45 R17s