Bought these in the 255/50-17 size replacing the stock 235/55-17 Pirelli P Zero Nero. This size fits the stock 17 x 8 wheels and is the same diameter so no speedo correction necessary. First off, the wider tire looks much better on this car than the relatively narrow stock tires. The sidewalls bulge slightly with the 8 wheel, but this rim width is well within the tire manufacturer's specifications.
As far as performance, these tires may not be as soft and sticky as the OEM Pirellis, but the extra width provides just about the same overall grip and maybe more. My car is a daily driver, but I have also taken it to 4 road course race tracks for HPDE track days. These Kumhos have been on the track at Watkins Glen now twice, for 2-1/2 hour sessions each, roughly 100 hard laps around the 3.5 mile track. Ideally for track use you want to use ultra performance summer tires, but even being all-season tires the Kumhos performed predictably and solidly. See lap here: http://www.youtube.com/watch?v=pzxOGIeYuqQ
After 30,000 miles these tires are about ready for replacement. The original Pirellis lasted 37,000 and I have no doubt the Kumhos would have gone that long too, if not for the abuse at the race tracks (probably the equivalent of 5,000 normal miles). For a high performance tire like this, that is an excellent life span! I'm planning on buying another set.