Makes all the accessories run 20% slower??? i think. which allows more power to the engine and less to the accessories. I bought one from mace, looks sweet havn't tried it yet though lol. Mate if you need anything talk to Steve he is the one to ask and a great source of knowledge and will help you out with installing any of these products, believe me ive been pestering him for a while now lol and he has done nothing but given me top quality service!
As for the diff ratio, and for someone chasing power, I DEFINATELY of all people recommend a new diff. I done it to my VT, 3.9 ratio, and believe me the ammount of pull it had after it was done was amazing...for anyone doing this specifically i recommend HIGHLY changing the gears. I got a tru trac fitted aswell, which acts as a single spinner untill you give the car crap, and then locks into LSD mode, and both wheels go at it. My speedo is sitting approximately 20kms off where it's supposed to be now lol...
Hope this helps you out mate!!!