I don't know about the underdrive set posted in the New Products section, but I had underdrive pulleys on a Mustang before and the charging would drop down at idle. The battery kept charged OK, but the headlights would visibly dim at a stoplight, and then get bright when you accelerate from a stop. This had the effect of looking like you were flashing your high-beams when taking off from a stoplight at night, and ********** many people when I drove the car. Plus it was embarassing to have the lights dim and brighten, like there was something wrong with the car.
All for a tiny gain in HP.
Of course I don't know if the alternator charging system in the Viper is that finicky, but the unavoidable downside is the typical "Viper tax" with the Viper underdrive set being $400-$450!, as opposed to $100 for other cars.
In fact I had read somewhere that one of the larger underdrive pulley manufacturers have alumunum pulleys for Dodge trucks that will bolt onto the Viper for something under $40, with 3 different sizes (percentages) available.
-Dean.