I'm in the market for new tires and i'm debating what tire to get. I wanted to know what everyone thinks would be the best performing street tire for auto-x/track? My 3 debates are the new Falken RT-615's, the Kumho MX's, and the BF-Goodrich Comp TA-KD's. I have 1 concern for each tire. Do the new Falkens get too hot for a track day like the older RT-215's? Are the Kumho's not sticky enough to compete with the other tires? And are the TA-KD's that much faster than the others to compensate for the price? And lastly, regardless of which tire, what size would perform better? 235/40-18, or 245/40-18? None are available in 245/35-18 BTW

In a test that was done here by Euro Tuner the MX was the best on the skid pad and the KD was the best in the slalom and braking. I do not have any information on the Falken you mention but there was a Falken tire in the test here that came in behind the other two tires.
You don't mention what car they are far so I can't recommend a size.
If I can help let me know.

Thanks for the tips, I went with the MX's mainly because of the cost. I didn't want to gamble with the azeni's heating problems because of how much more expensize they were, and the ta-kd's were just too expensive for the performance difference. I checked the hankook's and they don't have the size's I need. Besides, tirerack, had a special for 245/40-18's at $138 a tire, can't go wrong with that

You will like the MX. We will get them on the way.

strange that tirerack never has anything nice to say about Falkens... understandable since they're not a dealer :P
"some other falken?"

But I'm in the same boat at the moment. After running the MXs, the KD, and the Azenis(not the new 615s) on my Evolution 8, I've concluded that the KD's were FAR too expensive for a set of tires that would last me only a few months. On the track they gripped better than the MX, and didn't grease up like the Azenis until it got retardedly hot. But WAY too pricey for me at the moment.
(245)The MX outlasted the Azenis on the track(as far as greasiness is concerned), but the initial grip seemed noticably less(though still great for a street tire) than either the Azenis or the KD. Also, to note, this tire was my favorite on rainy days.
(245)The Azenis were amazing to me, my first few laps were identical to 911s running R compounds. The grip was equal to the KDs, and the exceptionally stiff sidewalls, combined with a lot of rear tire pressure really made that car handle. However like you said, as soon as they got hot enough, they just flat out sucked, talk about drift day... but again, that comes down to driver responsibility and me just overworking them. The other negative I noticed is they felt rather heavy.

The new 615's have apparently addressed the hydroplaning and some of the heat issues, though i don't see them being drastically different(hopefully foot in mouth).

I'm probably going to end up with the MX's like yourself. Mainly so I can save up for a set of track only tires/wheels. Seems like a waste driving these tires through 405 traffic everyday.