If you have pretty good tires already, a little bit of tire slip is good because it helps get the engine up in the power band rather than bogging. It's just that if you have to feather the throttle a ton to keep the tires from going up in smoke, the other improvements won't help as much.
I hope that is helpful with deciding on the order of things.
Don't worry about the other guys with their talk about the friend's light weight car. Yeah, a lighter car always wins if the other things are close, but you don't want to twist your car up with some crazy huge motor just to beat this guy. Let him put the SBC or whatever in there and then you can smile when his doors quit closing right because the body is a pretzel.