Interesting video from Tire Rack showing wider tires aren’t always faster.

At a certain point, you're introducing more friction than the power of the car is capable of overcoming. OEM tires factor this in, among other things. Generally, to the point of maximizing efficiency. Going to a wider tire isn't necessarily a bad idea, but you shouldn't do so drastically. 10-20 mm more of width will provide a noticeable bump in handling without becoming much of a detriment to performance IMO.