I hadn't heard of an underdrive pulley before, had to look it up -How exactly do underdrive pulley's function? how much power would I pick up if I installed one on my 4.0 v6?
Apparently it replaces the crankshaft pulley and the water pump pulley. I'm taking a wild guess here, but it looks like the new crank pulley is a tad smaller in diameter and the water pump pulley is a tad larger in diameter. I would think power pick up would be minimal, but AM claims 12hp. Am sure those more knowing will chime in.
If my crank pulley was defective (as it seems they go out often in our cars), I would consider the kit as the pulley looks to be a quality unit. Otherwise, I wouldn't bother due to the cost and hassle. But that's just me..