My EM car is pretty similar to your XP car, a bit lighter at 2300#.
I have trouble getting my R35 Hoosiers (25x13x16 front and 27x14x16 back) up to temp on cooler days or slicker surfaces. If you beat on them really, really, really hard on concrete in August with two drivers, you could probably overheat them, but I don't remember having to spray them on this car. If they made an R25 in the GT1 sizes, I would switch in a minute.
I ran CP for 9 years before building the EM car. I tried the R45's there and they were OK only if you had two drivers, but you gave up the first run or two. With the extra weight, you could overheat the R35's, but spraying kept them fine. You could actually wear the R45's out to cord, they just took a little longer to heat up as the runs piled on. The R35 (on both cars) pretty much turns to stone after 50-60 runs and is so hard that it seems to just stop wearing and the car is really bad.
I think Bob Ely in Colorado tried the Avons on his CP car..they were apparently amazing for about 1 run and they overheated and turned to grease. That may be perfect for our lighter cars, but Avon availability is a PITA and they are pricey.
I think with Goodyear pulling out, the Hoosier bias tires will never get any more development, at least not any that costs any money, there is just not a market there for Hoosier. That means that we are looking at the radial GT-1 tires which are hella expensive and also need a lot of work to be any good for autocross.
If my car didn't have Wide 5 and could accommodate a less than 27" tall tire on the back, I would be sorely tempted to try an A6.