Usually the alum. wheels will have less un-sprung weight on the end of the suspension, which makes the suspension react quicker. There really won't be much performance gains by going to Alum. wheels only. If you go to a bigger diameter tire, you will lose some low end grunt, but you'll gain a slight amount on top end, if you go to a smaller diameter tire, it will have a much snappier holeshot, but of course you'll lose some top end. Off set is the distance from the back or front of the wheel rim, to the mounting flange where the wheel bolts to the hub. More off set from the front means the wheel will stick out further (flange deeper into the wheel), more off set from the backside will mean the wheel will be tucked into the chassis further (this will usually result in the tire rubbing the tie rod ends).
Maybe this helps you and answers your questions.