Tires....

Nitto drags have a treadwear of 0. They have the best traction but they will last for about 6000-9000 miles. When my car was supercharged traction became a serious issue. I got the Nitto's and extremely pleased. I then got the Kuhmo' you're looking into. Traction not as good as the Nitto's but decent. Best part is they were great in all weather conditions. Since you need some mileage out of your next tire then get Kuhmo's.

What kind of 60's are people getting out of the kumho?

The wet driving thing would be a MAJOR plus. I have the BFG DR in 315 on mine and let me tell you being dependent on if its going to rain or not cuts down on the nice days you can really take out the car if there is a chance for showers later in the evening.

That kuhmo is a great autocross tire and there is a ton of information out there on it. From what I remember guys telling me that have them is that they suck in the wet. Being that the XS is made to be a class competitive autocross tire I have my doubts about you seeing 20k out of them but I could be wrong. I don't think the nt555 will hook very well in the non-drag radial variety. I hear nothing but great things about the nitto nt05 and there are a lot of mustang guys running them.