Sometimes I prefer to play on maps which aren't very deep. I've only done minimal investigation, but what I have found is extremely useful.
First of all, I generate a world with 1 cavern layer, which is set to low density and high openness. This creates a relatively flat cavern.
Then on the created world, I use dfhack 'prospect' to investigate the depth of sites. And here's the interesting thing, most sites, and AFAIK, all sites at the coast, are between 80 and 140 z-levels deep. But when you go inland, near the mountains, you sometimes find MUCH shallowerer land, the shallow sites are normally only about 30 z-levels deep, starting at about 27 z-levels. This is all pretty inconsistent, you can have the map quite abruptly go from 140 to 30. Some maps will have quite large areas of shallow land, and others quite small areas, or perhaps none at all (but I don't know a quick way to examine the whole map - a dfhack depthmapper would be brilliant).
So far I've only examined about 4 worlds (all have been based on small island), and once I found a ~30 site, I embarked on it and played. So my conclusions above are drawn from very limited data. But the most basic conclusion is if you generate a world with 1 flattened cavern, and hunt around with 'prospect' you can quickly find sites ~30 z-levels deep. Such sites are brilliant for magma stacks, and would improve FPS significantly.