Thats an almost 50% increase in voltage and therefore current - the resulting power applied to the motor will almost double. Whether or not an increase in speed is wanted, its going to be there. There might well be an improvement in low speed response because the ESC will be giving more powerful kicks to the motor which should start it turning earlier and maybe more reliably, but I suspect that it will not be huge.
Fortunately, Graupner have (had?) the reputation of solid products with generous safety margins built into their specifications, so as long as the loading is not excessive, the motors should handle it OK.
For the same amount of energy stored, NiMH are considerably lighter than SLA.