While I don't have a GP (or any L67/L36/etc.) powered vehicle, I do have an underdrive crank pulley on my vehicle.
It's about an 18% underdrive, and I do not have any cooling or charging issues, although when I run a lot of electrical accessories (like A/C, radio, headlights and cooling fan override), my ignition voltage will drop to about 12.4 volts.
The power difference I feel is negligible, but I have yet to obtain hard numbers to confirm.
