Consumer Reports did a test on Step ups. They used a Honda Accord and a BMW 5 series as test vehicles. Did samples of 17", 18", and 19". They like the result of one step (17"). Did not go through a tire sample, but used Z rated performance tires. They did a Tire Test a few months ago and really liked the Goodyear F1's.

Marginal gains for increasing to 18" and 19". Snow performance and rain (hydroplane) performance decreased over stock tires.

I put 18" on my car in the Summer and I can vouch for the dry performance. Live in San Diego so snow is nothing I need to consider.--ml