CPUs are designed with a certain clock speed in mind. The target frequency affects the design of literally every component. Turn up the frequency too high and signals won't arrive in time. Increasing the frequency to match performance with AMD and Intel might require an almost complete redesign.
Maybe it can run faster given better cooling and less aggressive power-saving. But I'd be surprised if it can be made to clock much higher, else Apple would have done it already. Mobile CPUs already have peak performance well above what they can sustain. An even faster turbo would be used more rarely but would still be useful.
All five sedans that beat the Model S around the Nürburgring drove their laps earlier. The Plaid has only ever been the sixth fastest sedan around the 'ring. Which is excellent, but not what you said.
>and faster than practically every sports car ever made.
It isn't. It loses even to the 2012 Nissan GT-R or 2007 911 GT2. It loses to any of the faster 911s by nearly a minute. It is faster than some sports cars, but not recent sports cars at a similar price.
How is it that the Plaid, despite being one of the fastest cars on the road in a straight line, loses quite badly to cars with half the power? Because there is much more to performance than just acceleration. The cars you disparage are dramatically better in the corners than the Plaid.
>The nurburgring is about the most sports car track ever made.
I don't quite know what this means, but I don't think it's true. The use of the 'ring as a benchmark is often criticized because it rewards power over handling. A track that specifically tests handling would be slower and twistier. This is not sour grapes; people have been saying this for decades.