Note: Even though the kit is offered now without a power system, the gearing of the heli hasn't changed. So whatever system you choose, the motor's kv still needs to be around the stock value (380kv or there abouts)...even if you plan to "sport" fly it on 12S.
When "sport" flown on 12S, the head speeds used are down from the 3D speeds of 1800 (give or take), down in the 1400-1500 range (3Ding the model on 12S will be a disappointment). So the drop in cell count from 14S to 12S makes this lower head speed happen in due course, even with the stock gearing.
Pinions are limited to three (17-19), and are all 8mm, so your motor needs an 8mm shaft as well.
The problem will be when guys want to "sport" fly the model, and drop a "standard" 700 class 12S power system in it, and they will find the kv value of their "standard" 700 power system motor is too high to obtain the lower head speeds 12S demands.
Guys then might be tempted to just run the heli at a higher head speed that the higher motor kv provides (on 12S..like their 700 class heli the system came off of). Well, they will then be trying to fly an 800 on a 700 power system.
Not going to work very well. I just don't want to see guys spend bucks and not be satisfied in the end is all I'm trying to avoid.