old#7 - I bought a heavy (just over a ton) Jet 13x40 four years ago and moved it into my attached 3-car garage/shop. I helped Dad form & pour the slabs for the floor in this garage, so I know they're only 4"-5" thick; we used steel mesh for reinforcement, which is much lighter than rebar. I had to re-level the Jet several times during that first winter, but the floor eventually settled to the point where I only needed to tweak it a bit with the changes in seasons. The last couple of years, it's checked out nearly perfect with an 98-8 Starrett machinist's level, with no adjustment needed.
This past fall, I started shopping for another lathe, and wound up purchasing a PM1440HD, which is nearly identical to the Jet, but weighs about 2500lbs. I set it up back-to-back within 20" of the Jet, which of course resulted in causing the Jet to move as the floor settled to accomodate the new lathe. I'd have been better off putting it farther away from the Jet, but my mill takes up the next bay over, so this was the best spot I could come up with. The floor seems to have settled already, as the last time I checked both machines with the level, they were spot on.
However, I'm more concerned with vibration than whether the floor will hold the lathes level. If I were doing it over, I'd have rented a concrete saw, cut out an area for two lathes, and dug down far enough to allow pouring a fiber-reinforced concrete pad about 10" thick. This would address my concerns about vibration.
As far as raising the lathe goes, I know what you mean about saving your back. After several hours of standing over the Jet while doing a barrel, my back feels somewhat strained. The PM sits just enough taller so that back strain hasn't been an issue. Or maybe it's just that I'm more relaxed while threading & chambering as I've gained experience, and it was tension that caused most of the back strain? Whatever - both these machines have foot brakes, and since I learned on them, I'm used to having the brake. I'm using the brake on the PM on a regular basis, as I've got it set up to cut metric threads. I'd hate to do anything that would make it harder to hit the brake in an emergency.