Thanks for the observances, John.
It's been a while since I had IRCs on my scoot, and it was original equipment (and I was new to riding), but I'm pretty sure my rear IRC wore a bit square. I remember thinking the handling had deteriorated, but was clueless (at the time) as to why. New Diablos made it ride fantastic for a while.
The one tire tha claims to be made to wear round is a Michelin Power Pure. It seemed to retain its handling longer, but I got less mileage on it than anything else I've used. If they made both the center and the sides from harder rubber, it would have been an excellent tire.