As planned, I went ahead and installed two new power districts over the course of the week - both of them were installed on the visible main line. Since the visible part of the layout is at the top of the grade, it requires the most current to keep trains operating. Locomotives draw more and more current as the entire consist hits the grade. Upon reach the summit at each end of the upper level, though, the locomotives must continue on level track while pulling the train up the grade, and so still draw quite a bit of current.
Installing two new power districts, each with its own EB-1 breaker set to 5 amps, I seem to have eliminated current-draw power interruptions. With two heavy trains at mostly open throttles, 3 Athearn units each, working up 2% grades with 35-40 cars, I had no issues - a nice relief, given the prospective cost of adding another booster to the system.
Soon, I will be installing headlights in my older Athearn units for the first time, and will also work to finalize the design of the engine yard. Winter is a great time for model railroading!