So from what I have read on the forums it seems that the 17" wheels are the best for racing or performance purposes (correct me if I'm wrong ). However what do I stand to gain (or lose) by raising the tire width other than traction?

Also whats the largest with that will fit on stock suspenison without rubbing?

 

How wide you can go depends on how your suspension is set up and whether the car has been modified to make the wheels fit.

As for the 17" wheels being the best size for racing and performance...no. Again, that depends on the way your car is set up. There is no "best size" for all racing applications. That is why various race cars use different wheel sizes.

Wider tires and wheels can gain you traction, but cost you unsprung weight, braking, acceleration, etc. Usually, the extra traction is considered worth the trade offs.