There are some 10Mhz Enterprises. Actually this is the upper limit, not a problem of the Z80, but from the other custom chips and the memory.
On the other side, the speed of the game depends on which of the several clocks are used inside the game. If the video frame rate is the timing used, you won't notice any change on the game play when overclocked.
Actually the Enterprise is being implemented on a FPGA, and it will allow higher speeds on a near future.
Said that, this is not a competition....
