In recent testing I hit 836,160, but getting it that high, during a speedrun, would be insanely difficult.

I don't know much about the run/route yet but interested/interesting. Maybe with some routing it could become more feasible?

This maybe 3 years old, but, can anyone beat my 920K?

i really have to check tricks and stuff for this game. how are your buildings even able to grow when the power sign is still flashing on them? i only played the german version and i don't know the differences. :D

That is a glitch within the game that happening when you are building anywhere above 875,000 population. It will also happen if you rezone too many times . For example:
If I bull dozed a Commercial Zone 4 times and built something in its same spot, then the power being consumed by those previous zones does not stop, so essentially, there is 4 zones hogging power in just one zone. As for the other Glitches that happen with power:
The reason I can still grow with power, is it will mechanically be powered but visually not when you start growing beyond 875,000 Population and that is due to the game engine not being able to correctly process the balance of electrical power. It will also Start doing this below 115,000 if you are zone stacking. 

This glitch will also occur if you do not built power lines. Technically you do not need them, however, the game engine will get confused and sometimes think that your zones are not connected to power even though they are. This will sometimes never go away, but  if you do have the actual ratio of power plants to zones; it will only become visual.

Power Scale for refencing (not perfect but seems consistent enough to use) - 
Power Plant = Zones Powered (difficulty)
Zone Type = Number of Zones worth of Power Consumed:
Coal Power Plant = 60 zones (Easy)/50 zones (Normal)/40 zones (Hard)
Nuclear Power Plant = 84 Zones (Easy)/72 Zones (Normal)/60 Zones (Hard)
R-Zone = 1 Zone
C-Zone = 1 Zone
i-Zone = 1.5 Zones
Police - 5 Zones
Fire - 5 Zones
Presents - 1.5 Zones
Power Line - 0.125 Zones
Sea Port - 15 Zones (Easy)/30 Zones (Normal)/45 Zones (Hard)
Air Port - 60 Zones (Easy)/70 Zones (Normal)/80 Zones (Hard)

Air Port and Sea Port are the only 2 buildable that consumption of power changes on your difficulty.

I Hope this was Helpful.

wow thank you for the very detailed explanation!
i'm super amazed by how these speedruns look like.