I have tried the Kumho XS and Dunlop Z2 (old version)and BFG Rivals back to back on several occasions, on different surfaces...all on my HS Mini Cooper.
I have had my share of mixed results. But here are my results...in no particular order.
The Rivals are capable of turning times equivalent to or at least comparable to the other two, but feel like crap doing it. All crispness of a Mini Cooper turns very numb. Turn-in on Slaloms are very slow and mushy.
Z2's heat up very quickly. Good for cold temperature days. But with two drivers or even one driver on a hot day...get way too hot on a Mini Cooper. These become a one or two run tire. Each run after that get slower and slower. Again...they are capable of turning very fast runs...at least once before getting too hot. Ultimately the faster of the 3, but they have a hard time duplicating it back to back due to overheating. Definately not a Prosolo Tire.
Kumho XS, was ultimately the tire I chose to run, because they could take the heat that a Mini Cooper dished out. They could do that consistantly, and during back to back testing, even if a touch slower. They were ultimately fastest on second run(once they had a bit of heat) but could back it up on subsiquent runs. They handle two drivers just fine with light watering. They handle hot days just fine with light watering. They feel great and inspire confidence. They are crisp, turn in is great, and sustained grip is good in sweepers. The downside is that they are fastest after shaving to 5/32"(extra expense) and they seem to have a base rubber that isn't as fast. So times start to go away around the 2/32" mark. This makes them good for about 25-50 runs. Not much different than I got from Hoosiers. But the good news, is that their contingency program is awesome, and after my first set last year...they provided me with my next 10 sets for free(other than the shaving expense). Plus I still have 12 more tires that I earned at Nationals, that I can't run next year.
Oddly, I ended last year with 19 Hoosiers that I couldn't run this year...I hope these rules stop changing soon.