I have the Steeda UDPs on my '03 GT, including the generator pulley, and have never had any charging problems. As to increase in fuel economy it will be minimal; 0.5 mpg at very best.
The most noticeable difference will manifest itself on hard acceleration in the lower gears, especially over 4000 rpm. If you are not the sort to do that you might want to reconsider spending time and money on UDPs...
- 2003 GT, UPR X, FRPP 24lb/h, Magnaflow, PP 70mm TB & plenum, Delta Force tuned,
Steeda UDPs, Ralco flywheel, RAM HDX clutch, 3.73s, 262 rwHP/305 lb-ft.
New ride (7/1/2013) 1998 Mercedes SL500-5.0L 32V VVT 326/347 HP/tq