Emilio from 949 Racing posted a pretty good review of these tyres on miataturbo.net so I thought I'd quote it here for those that don't frequent that forum and would like more information from someone who has used them in anger:
04-17-2013, 03:05 AM:
"We had the chance to run the 225/45/15 Rival on Buttonwillow 13CW last Sunday. Car was our 95R street project car. Xida 700/400, 15x9's, 140whp, 2550# (with passengers all day). Ambient was 75~85° with track surface temps of 93~106°. We have run this car with RS3's on several different tracks and weather conditions. Very familiar with min corner speeds, braking points and overall feel of the RS3 on 13CW in particular. No A/B testing done against the RS3's Sunday but some impressions stood out.
SOTP, the Rival seems to generate about the same peak lateral grip as the RS3. The difference was that the Rival would still generate useful grip when well past optimum slip angle. More like a race tire than the RS3 which prefers small slip angle and loses grip more rapidly with large slip angles. This made the Rival easier to drive and allowed a few "toss and catch" entries into double apex turns that were a touch faster than the slight lift the RS3 required in the same spot. This ability to generate useful grip at large slip angles made them feel more like race tires than any other feature.
Response and feedback were, like the RS3, very good when the tire was lightly loaded as it would be entering a turn at full throttle. The noticeable difference was under straight line or heavy trail braking where the Rival communicated available grip and slip angle much better than the RS3. The RS3's can get a little numb under heavy trail braking. So unloaded they steered about the same. Transfer weight onto the nose and the Rivals talked more. Overall I would rate the steering response and feedback as very good.
This is where the Rival is significantly different than the RS3. The RS3 has earned a reputation as a tire with braking that does not quite match is stellar lateral grip. The Rival had noticeable better braking grip and feedback compared to the RS3. Of note is the RS3's characteristic of requiring the driver to transfer weight onto the nose more carefully while the same car/driver on Rivals could reach peak line pressure much sooner without locking.
Since we were not conducting A/B testing that day, data collected was minimal. Peak temps were only in the low 170's even after a full session. We did see that the Rivals lap times stayed consistent even after several laps at full speed. This is rare for a street tire. Most street tires have slightly heavier casings and more tread mass than a race tire so they tend to not shed heat as well as a race tire. This typically manifests as the street tire being fastest on it's first hot lap. While there was some drop off late in the session it was less than .6s on a 2:05 course. This is excellent for a full tread tire. The indication here is that the tire sheds excess heat very well, possibly better than most other tires in this category. My guess is that autocrossers with a co-driver will fare better than solo drivers on this tire. We did not run the tire in the morning while it was still cold so I can't comment on cold performance. We ended up with about 35psi hot which compares to 29~34 psi we usually run with R compound tires.
Overall, the RS3 feels like a really fast street tire, having a narrower range of optimum slip angle and weaker braking than a true R compound. The Rival in contrast, felt quite a bit like an NT01 (R compound) with about 5 heat cycles on them. An educated guess is that the 225/45 Rival would be less than 1s slower than a 225/45 NT01 on a ~2:00 road course, everything else being qual. That is just astonishing to this driver. A 200 treadwear (grain of salt) that can nearly keep up with medium compound race tires for a full track session."
for the thread (Emilio's comment is on Page 2).