I just wanted to mention that thermostability is likely going to be attributed to multiple factors - that is, it's not going to be X, Y, or Z, it's likely going to be all of them (and other factors!) contributing. I can cite one example off the top of my head - the tungsten-containing aldehyde:ferredoxin oxidoreductase from P. furiosus had a greater number of residue-to-residue contacts than expected, as well as tighter "packing" and seemed to bury surface area more effectively. (If you need an informal reference, it's from the Rees lab at Caltech from the mid to late '90s. Otherwise, I'll have to look it up.)
Also, I would keep in mind that even something like P. furiosus tends to top out at ~ 100° C in terms of its growth behavior. It's not as if you can keep cranking up the heat and expect it to be fine! (I recall that its pH preferences are fairly well typical - prefers fairly neutral conditions but can tolerate a pH unit or so in either direction.)