As far as a straight swap, I wouldn't expect much.
With them adjustable, you can adjust pinion angle to get more "bite" and help 60 ft times. Note this more aggressive pinion angle probably isn't gonna help your U-joint life.
All else being equal, a car with extendeds will be slower than the same car with stock length. Geometry gets worse, and weight distribution gets worse (i.e. more front heavy) with the extendeds.