I have never seen a real dyno graph showing more than 9 rwhp with just the addition of the underdrive pulley. That doesn't mean no one has, but I have yet to see one, and I've been looking for 5 years.
As far as the topic at hand, how come no one has said nitrous? Biggest gain for the money hands down. Still like said I would upgrade to a 6.0 first.