15" wheel package comparable to 14"

That's about as close to the same rolling diameter as you're going to get with 0.6" more treadwidth per tire. Good call from Summit. As you can see from their recommendation that the first number is the section width of the tire, not the tread width.

By going with a nearly identical tire height, ride height of the car will be nearly the same. If your speedometer has been reading "fast", this should "slow it down" a little. i.e. If your speedometer reads 60 and your car is actually going 55, the change should bring your car speed closer to 60.