I don't want to start any rumors, but the SGX CPU did have an "A" revision that the other systems did not.
Although unlikey, it's possible that there would be slightly less slowdown - maybe a reduction in opcode penalties (+1 cpu clock cycle wait state is added to every access/read/write of the VCE and VDC on the non "A" revision hu6280 systems, even though it is not needed - info from Charles MacDonald).
By comparison, some roms running on MagicEngine, slowdown less than the real machine, even though the CPU is not overclocked, because certain penalties are not emulated. So it's possible that the "A" revision has reduced clock cycles on some instructions and/or does not have the VDC/VCE +1 clock cycle penalty like its brethren.