i used kumho mx for the last 3 years and recently mounted the spt (all in 245/40/18). the mxs i drove 6 sessions at laguna seca and a bunch of autocrosses and the spt, no racing/track yet.

the mx kept working well until i had worn well into the steel belts. ow they can stick you good. they were never very good in the rain. i got about 15K miles which is pretty good.

the spt has a softer sidewall and an easier ride but are not hugely quieter. the grip is a bit less, even than the well used mx, which surprised me.

the mx resisted heat pretty well and stood up to the day at laguna seca even though i cracked a rotor, warped all four and dissolved my pads.

ive heard the spt is a very good rain tire but no rain yet this year where i live.

so far i think i probably should have stuck with the mx. neither are suitable for snow.

hope this helps someone!