I've been on an anti-lag quest for a while now, as my machine is much older (2.45ghz single-core). The main difference between my forts and what you've discussed is that I try to limit access to a single path between any two points, whereas your bajillion stairways would be the exact opposite philosophy. My 'single path' can be several tiles wide, sure, to help avoid the traffic jams, but I try to ensure there's only one overall route from any given room to any other (central staircase, each floor having several hallways with rooms hanging off them). While plentiful access routes would yield the shortest trip times, that's not what we care about - we care about pathfinding time, which cares about evaluating potential routes. By restricting the number of viable potential routes (augmented ideally by good traffic designations and wall placements to get the pathfinding algorithm to those routes sooner), we cut down the pathfinding time, yielding better DF performance, even if per-dwarf travel time suffers.
I'm still unclear about the effects of wall-less designs. I usually have one or two floors that are largely open space (a storeroom and my dining hall, which is basically a prepared meal storage room), with the rest the hallways-with-rooms design. Expansion of those open rooms never seems to cause me fps issues, unless I add complex stockpiles along with it, which makes me blame the stockpiles, rather than the room design. While currently the rooms of hallways-with-rooms /only/ open onto the hallways (usually through a 2-tile gap), in my older forts I'd have room-to-room opennings along shared walls or at the corners. I had thought the many opennings would ease travel and improve things compared to the heavily-walled environment it came from, but instead it would very reliably kill my FPS. So while I'm unclear on if highly-walled designs (which seem like they would be inefficient for pathfinding) or wall-less designs (which sound ideal, pathfinding-wise) are superior, the middleground between the two seems to be the worst place. So I aim for low-complexity walled designs in general.
From your description, the many-routes stairwells seems to be the primary difference in fort design - I stick with 2x2, ~30 dwarf forts, with 1 cavern layer, about 50-60 z-levels (14 sky + ~3 soil + 5 above + ~10 cavern + ~5 magma + ~15 hfs, if I remember right) and close-to-flat terrain. If I ignore magma and waterworks (aside from setting up a magma-workshop area via tunnels fed by fortifications, ie down at the magma layer), and keep the total animals low (<50), I can get 30-40fps, though I usually have trouble keeping animals much below 200, and the slowly accumulating item stores (even with atomsmashing goblinite) bring me down to 15-20 fps pretty easily when I'm not hypervigilant. When forts start getting into the 4-10fps range, I usually give up. Currently, I'm in a 18-22fps fort, my first goblinite-smashing one. It's held up much, much better than I'd expected, compared with 31.18 & earlier forts.