sounds like the right belt, i believe the A4 is the heavy duty, like they should make anything but a heavy duty
To get more speed you probably need to tighten the belt more by adjusting the rotor gearbox. Loosen the four bolts that clamp the box in place some, tighten the adjusting rod that is above and to the left of the box, the nut is placed so that you will probably scap your knuckles on the oil cooler. when you get it adjusted tighten the clamping bolts back down and your done. your owners manual should have a description of this process, and how much you should tighten.