Underdrive pulleys do not free up much horsepower at all. Most companies say 10-12 but in reality it is generally about 3-7 or so. The link below was done on a 6.0L 2500 Series with ac. That claim is pretty accurate.
Here is a semi-relevant dyno sheet from my Splitport 94 LX. The difference between the two pulls was that on the blue one I had installed a 25% UD crank pulley, swapped from the stock 94 exhaust to a much better flowing setup (click here for info on that), and tuned for 93 octane as opposed to 87 octane.
Keep in mind, that was back in 2003. If I felt like messing with the tune, I'm sure I could get more power out of the car now.