Could you double the speed of a GPU by doing half the work?
Probably not.

First of all TDP doesn't linearily scale with clock speed, a doubling in clock speed might triple or quadruple or even further increase the heat produced. 

Secondly, heat is not the only limiting factor of clock speed. The signal still has to reach its destination inside the IC within a limited time and the logic gates and traces add latency. If you increase clock speed enough, without changing the GPU physically, the signal won't travel fast enough and the logic circuit won't function properly.