[MetalMan]

Just to add this, the XL4200 and the MM 4600 motors should draw about the same current, the 4600 a slight bit more.
If you go by Feigao's turns, the 4600 could be like the loaded kv of a 7s.

The only way to get longer runtimes is to get batteries with higher capacity (Lipo) or go with higher voltage. For higher voltage, extra NiMh cells would add extra unnecessary weight - Lipo's the option for high voltage as well. For 6 NiMh cells, you're going to have a bit of a hard time coming by a motor that's powerful enough yet light enough at the same time - a Feigao 380C 6L would probably be the best choice for 380 motors (and that has a similar kv to the MM 4600 motor).