I have been using NiZn AAA batteries for more than 9 months. I have been using them on all 4 of my Mini-Z, with stock FETs and added FETs, various motors from AD Stock, PN50, 33, to custom hand-wound motors (28T). All of my cars and motors are still running fine without any problems at all. Due to higher than standard voltage (nominal 1.6V with peak 1.9V), batteries and motors will be quite hot after running. Whether it will be overheated depends on various factors, one being the gear ratio.
C-Hing, you don't want to use higher voltage batteries on your Transmitter. Instead what you want is batteries with more capacity (mAh). For example, Sanyo/GP/Panasonic/Powerex NiMH batteries. Higher voltage batteries are for motors, lights, mechanical devices that will gain more power with higher voltage.