UP can be felt as soon as you accelerate, I wouldn't put gain numbers on it but you can feel the difference easily, that usually isn't the case with other 986 mods where you aren't sure if it just sounds faster.
As for cost, here's the one I had, might as well replace the roller bearings and you need a new belt so add about 40 extra.
RSS Underdrive Pulleys - RSS
Threaded Mode
