wide rear wheels?

Im looking for a new set of wheels for my mk1 16v. currently running p slots which are 14 x 6j and i would like some wheels that are 15 x 7j or 7.5j.

I want the wider wheels for the traction and handling.

Anyway my question is this:
I see alot of show cars with wider rear wheels but does this improve the cars handling in anyway or does it do the opposite?

I know it cant improve traction with it being front wheel drive

I think the wider rear wheels are just there for looks. I've got 14x8 on the front and 14x9 on the back and the car definately prefers to understeer when pushed - it handled much more neutral with the bbs ra's on. Although it does grip a lot more now