Is 12.27 finally stable enough to upgrade from the original firmware or should I keep waiting?

You skipped the firmware upgrades for several months because of????

Well, we need to know what have been holding him/her back from upgrading.

Claims of poor battery performance, hrv and training status. The original software did this well so I stayed with it

Im on 12.27 and as information into your decision:

I use about 0.5% per hour generally and 6% per hour if using activity with full Multiband. I am connected to IOS device all the time.

HRV and Training status are bugbears of mine and my HRV has bene trending downwards recently and I can't figure out why. If its me or soem other factor Im not sure het. Training status is better for me than with 12.23 12.25 but still not in line with what I recall from my 745 etc.

Normal day to day usage and activity (Run) usage has been ok for me. 

If you use HRM pro or Dynamics RUN to run forget it. Heartbeat is bugged. Not using it for running either, in my case it broke the heart rate sensor.

As far as I can see, battery problems was mainly due to people not rebooting their watch after a FW upgrade.  Yes, have been a few examples of huge battery consumptions but problem seemed in limited numbers and it was not possible to identify the cause (I, myself have experienced this once).

HRV and Training status is related to one thing VO2-max not being updates. I am not sure if that has been fixed in this release. I have only been on the FW for a few runs.

I don't think so, my battery life is still 1/3rd what it was with the 11.x firmware. I wish there was a way to go back. I'm really jealous of the people that never updated.

Turn off your watch, switch back to a stock watch-face and I bet you wont experience bad battery issues unless your watch is faulty.