I checked out the Visplane Explorer feature in GZDoom Builder. I don't know much at all about visplanes, but I guess it roughly means the amount of stuff that is being rendered onscreen by the engine. Since the values it shows are X/128, I'm guessing 128 is the vanilla limit?
There is just one little spot by the eastern window overlooking the central outdoor courtyard which shows up as red for me, with values going above 128. I was thinking about possibly optimizing this for the sake of consistency. I tried removing one of the window sectors on the other side where you can see part of the inside room, but that didn't seem to make much of a difference, so I'm unsure what the proper way of doing things is here.
I'm not even sure if it's worth bothering. If it's saved in Boom format, it's not going to run in anything that doesn't support it, right?
Also I'm getting better at creating stuff without a bunch of unwanted artifacts now. The best way of extending room layouts seem to be to draw the new sector, then select the sector it's connected to first, then the new one and merge them, so it will inherit the properties from the sector that was selected first. Makes work a lot easier in many situations.