I have underdrive pulleys and there is a small noticeable difference - mostly at the top end (you will need a new belt). However, I think a crank pully would be a better buy - I think you'll see a smidgen quicker acceleration (I think you can still use the same serpentine belt with the crank pulley).
However, if all you have is $500 (equivalent to pulleys & installation) to spend then I suggest an ECU tune - see TVT Design for that (they've just done mine and their customer service is top-notch).

Regarding the crank pulley - do a search on this forum - somebod posted a thread about an MB with a badly installed crank pulley - that scared me from installing one.