I haven't had any problems with not tempering. I don't know if tempering is necessary since this is case hardening and only the very thin surface layer is hardened. The bulk of the part should be low carbon steel and therefore does not harden.

I also haven't had any warping, but it could be a problem especially with larger parts.

I am not familiar with the powder coating ...maybe using it does protect it somewhat, the part would be more prone to warping if you went directly from heating to quencing in water. When you quench in oil it lowers the temp slower decreasing the chance of warping and cracking, oil is also a good source of carbon

The parts I had hard coated required bearing fit as well. It may have made things a little tighter but my machinist compensating taking off an extra few thousandths. Probably a little touch up with a scotch bright pad would fix in problems though.

It will still harden,not as hard as the surface,but it will still harden. If you cut the gear across and measured the hardness, it would decrease as you get twoards the middle, but you'd still be harder than before. Also, they probably don't spend a whole lot of time and money engineering and machining those gears to exacting tolerances, and your probably going to have a lot of imperfections that would be a hot spot for bad stuff to start happening at the surface if you don't temper. And yeah like the metallica guy said, use oil rather than water. Water is more severe of a quench than oil. It'll be just as good, but will have less of a tendency to crack. Im pretty sure warping that small of a gear isn't an issue, but still it will in fact keep distortion down more than water. I think if you temper your gear at 400, you will relieve the stress while not sacrificing too much surface hardness. It will fail prematurely in the as quenched condition. I've never seen anyone use anything untempered without it cracking/breaking. Even 300 would be better than nothing.