I'm not actually looking for sheer cliffs, just as much height as possible.
After messing around with the world painter a bit more, it seems that the elevation brush doesn't just paint a fixed elevation, it paints a range of possible elevations. Entering a brush value actually results in two values, displayed as, for example, 200/125; the first value is the one you entered, and the second value is the same as the first for 100 or below (i.e. underwater) and lower when above sea level. Although I'm too lazy to figure out an equation for this it seems that the difference between the two maxes out at 150 for values above 300.
I imagine this has already been worked out by half a dozen threads a long time ago when the world painter was first created. And something about that paragraph... doesn't seem quite right. Maybe it's because mountains start at 300 and a wodge of 400/250 elevation should contain some not-mountain by my logic, which it doesn't. Hmm...
Anyway, it's not going to be easy to get the kind of mountains I want; not on an embark with a reasonable horizontal size, at least. Might be able to get something decent by putting some high mountains next to a deep ocean, though.
EDIT: Got a spot with a mountain 86 levels above the ground and an ocean trench 69 levels below the ground, for a total of 155 or so. The ocean seems to be so deep as to extend into the "fog of war" in the cavern regions, though, so I don't think the ocean counts toward giving me more total room. At least I don't have to worry about having all that water on my map now.
I suppose that map would make a glorious screenshot with a 3D visualizer, though. Had some neat cliffs and plateaus. I'll store that one in a safe folder for later so I don't actually delete it while I wait for Obsidian to be finished.