Hi dfwjeff,
Some of the best sources for a latex (or other) toppers I know of are listed in post #4 here.
This would depend on what you were comparing it to and what the intent of the Xfirm layer was. the lowest layers have the least effect on the feel of the mattress but if you used Xfirm instead of firm then it would be slightly more supportive. If you added an Xfirm layer to the bottom of a mattress and increased it’s thickness … then the extra layer would be softer than the floor and the mattress would be slightly softer and the thicker layers would compress a little more gradually (the mattress would have a more gradual compression from soft to firm with compression which can be an advantage with greater weights). As you mentioned it can also help fill a loose mattress cover but if this was the only intent I would probably use very firm polyfoam.
It’s not ideal in terms of ventilation and humidity control but for the short term it should be fine. I would put a blanket or something on the floor under it for protection though to prevent soiling or stains and I would make sure it had a mattress protector over it.
My personal opinion is that slats are a better surface for a mattress but there are differing opinions about this. Post #7 here has more about this.
Phoenix