Mazdatech is blowing smoke up your... If they denied warranty under the excuse of "excessive speed" they'd be sued out of existence in short order. The only way that could be enforced, if the car had a speed limiter and that would be inactivated by the owner, plus(!) a specific note to that effect in the warranty documents. If the car is capable of the speed out of the factory "as is" then the warranty is enforceable. Local speed limits (if any) are irrelevant.