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is there a general consensus on when and how the RT615's lose grip?  I heard a few times, from respectable sources, that they can and will do that, but how do I know that this is happening? 

 background -- I've been running a set of RT615's last year, and was lighting up the inside rear a lot, probably heatcycling the rear tires quite a lot, and wearing down to wearbars in just a few events.  The issue got a bit better after turning up the front bar (1" bar, MR2 spyder, with 205/50/15 RT615 all around), but didn't quite disappear.  This year, with new fresh rear R615's (and old fronts) the car understeers resolutely, and the inside rear does not spin no matter how abrupt I am with the gas pedal.

So, the question, is -- how do I tell that the tires are beginning to "go away", is this a number of heatcycles thing, or should I change them before they get to wearbars, or is there any other way to tell when they need to be changed?

Swap em front to rear and you have your answer.  :)  I always had good experience with them not losing a lot of grip down to the cord.  But then again, I changed them a full set at a time.  Probably heat cycling if anything. like you said.  Do you rotate front to rear each event?

that is a pretty good test, except my wheels are staggered-- 6" in the front, and 6.5" in the rear.  rim width might affect the amount of grip, afaik.

 I guess I can keep a set on fresh tires mounted on spare rims, and just swap them for testing every now and then...  I think I will do just that!  thanks, Patrick!