Although the V-reactor is efficient on it's own, and expandable in it's own way, I could imagine a proper building of the Tower of Power Hydro-Plant (I guess dividing it into separate buildings (5x15 WW/2 floors area (7500 Us (7.5kUs) per-floor 15,000 Us (15kUs) per 4-floor stack proper) linked by a small set of mechanisms/axles to connect power) can get really tall, and really powerful in a relatively compact cluster of buildings; 1 local embark tile at most with this build. At least the notes I jotted down allows the OUWR to be a stackable design, and like also noted, having a ramp system or corkscrew can regulate the water on it's way back down to the bottom floor again without having it evaporate, thereby easing up FPS a bit more as well as having more direct control over it, and maybe also double it as an aqueduct system if you want.
I'm building my own version of that design to see if it can work as I want it to, and later on, if it can really be stacked. So far, in theory, it should work. Essentially, the bottom-most floor (and if much much larger, unless longer more WW rows-wise) should provide more than enough power to jumpstart it once enough water flows in, or is even lightly pumped from lvl.1 and 2 of the reactor.
The guy that made this originally proved it worked, but the stackability has yet to be put into practice, and to be proven as efficient and FPS-safe. From what I can observe, especially regarding lvl.3's clustering of WWs, this build has potential to be really efficient if done right. My kill-switch placement advice is just in case things go ass-end-up. The power regulator is also how to better use the power output in a schematic fashion, using as few levers/gear mechansims as possible if you need to toggle the power, or alternate the power use (if per pressure-plate use).
EDIT:
Fixed measurements. Of course, I'm going by every U (Urists) = w (Watts).
With proper measurement, you'll be needing lots of floors, and maybe multiple reactor buildings to link up. I mean, 1.21 * 109 = 1,210,000,000Us or 1.21GUs. That is indeed a challenge. You can power the dwarven world from this.
I can imagine to maximize output efficiency (caused by water level dipping below 4/7), I would actually flood the entire bottom floor(s (7/7), if multiple buildings; or really tall, where it would be 1 entire flooded floor every 4 floors/proper stack (1:4 ratio, 1:2 if excluding dedicated WW floors))) (while the main pump chains are off) for all power-plant buildings used, then re-activate it, and kick-start the machine by manually pumping the upper-most water-accessible point to begin a chian-reaction jumpstart to the entire building. After one building has fully-powered, then reactivate the links to the other identical buildings. The one building alone can jumpstart the rest of the other buildings (up to maybe 4-6 identical-size & parts) simultaneously.
And if you're the impatient type, just keep flooding it (from the end of the flooding pipe that goes directly to the pumps) as you pump it until you see an ideal average flow-level per-floor (4/7) at the center of each (water) floor.