Underdrive pulleys will basically turn your alternator/ power steering/ AC slower - at the ultra low idle of an svx you aren't cranking out much already. I wouldn't recommend against one - alot of people here have them with -so far- no apparent problems.
With the amount of computer controlled functions in the svx I do not want to risk starving the brain of its lifeblood and I like my AC as cold as it can be.
Although I do think the svx has too much power assist in the steering - maybe I can get a bigger pulley for the power steering pump and slow that sucker down.