To the best of my knowledge, Tamiya has never published or utilized a list of acceptable silver can motor regs. I know that TCS Canada uses a formula for determining acceptable maximum performance, and the US Nats did the same thing for black cans: at 5 volts, no more than 2.4 amps and 19,000 RPM. There were lots of fast cars, but I do not believe that anyone was disqualified for exceeding those performance standards.
Even though there are genetic mutant silver cans out there that have all of the specs stacked in their favor, most of them will do 14,000 with good torque at 5 volts without much work. And there are all sorts of motor run-in jigs that people have put together that simulate actual motor use. In fact, the one on my test bench right now is in its 4th year of use, even though it was brandy new a few days ago...except that the conditions are much more conducive to long life than a dirty old race track.
And, just to be clear: I hopelessly overmotor, but I do not crank.