Blueprint Preview (Thanks /u/demodude4u for the amazing Blueprint Bot!):
The ratios are NOT perfect (horror!), but they're as close as I could realistically get them without compromising the design. I wanted everything to fit in horizontally tilable columns that consumed the equivalent amount of energy to the reactors directly beneath them. I eventually settled on a 9-tile wide column design which produces 279.36 MW of power. 9 tiles worth of reactor is theoretically capable of outputting 288MW of energy, so this is a very healthy chunk of that (97% yield). Here's a close-up of the reactor layout and heat-exchanger portion of the column design:
I'm quite proud of the water unbarrelling setup which I'm using here:
Since the water and steam pipes have to have the same throughput (1 unit of water = 1 unit of steam), they both suffer from the same dropoff bottleneck. Every pipe which I added caused hundreds of units of fluid per second to disappear. One neat thing which I've learned about pumps is that they don't suffer from any dropoff. Using a straight line of pumps will always preserve the throughput. It turns out I couldn't afford to even add a few underground pipes to make way for power poles in one of the existing rows of pumps. That's how down to the wire this design had to be in order to maintain efficiency!
In the end, it did end up meeting the 20GW target goal while maintaining horizontal scalability:
Edit: Fixed spoiler image tags