Bill I used the same plugs in a much higher HP motor the was regulary taken to 9000+ RPM. Did you happen to see the blue and red RX7 at the ARRC? That's my old car and I built that motor 7 years ago and it is still going strong, all the time using the 10.5 plugs. I don't think the plugs are your problem.

I would guess that when the motor was rebuilt the apex seal springs were not replaced and worked just fine at idle but at higher RPM they were too weak to hold the seal against the housing. Result is that the seal bangs around in there and eventually hammers itself to death. That old seal wasted your $30 plugs and most likely the housing too.

You might want to think about a different engine builder. Sorry you had a crappy race.