Neu motors on higher voltages

[crazyjr]

I know there have been discussions on this subject, but haven't seen much real info. I'm wanting to experiment with higher voltages (8s and higher), But don't want to drop money on a 1000 to 1400kv neu yet. I know my 1515/2.5d (1700kv) will handle 8s on RPM (57,120 using 4.2V max charge per cell). Geared properly, Will this be more efficient than the same speed on 4,5, or 6 cell Lipo? The planned speed will be around 45 mph (racing and bashing), Would I be better off just getting a lower kv motor?