I just wanted to know for the people who have the underdrive pulley's is there a big gain in horse power? And I have the shaker 1000 in my car and would like to know if there would be any charging problems? What does it slow down and how does the car drive and feel?

 

I have no personal experience with UD pulleys on an 05 but as a general rule of thumb:

UD Pulleys and a high performance sound system do NOT mix. You can correct this, however, by installing a counter-active pulley on the alternator.
A/C is generally not hindered to the point that you might feel it.
Power steering is almost never a problem.

I know I may not be extemely helpful with this, but I hope it does something for you.