.... bad .... very bad
Here's a quick tip for those of you going through this issue.....
I checked it out yesterday and there seems to be no lift BUT I have not played for long stretches on it and nor did I overheat it too much. Fact is the phone is sealed so it does not dissipate heat all too well.
Remedy?
1. When using it for intensive things, it makes a lot of sense to use it flat on another surface to help distribute heat ... for example a glass or metal surface. . essentially something that feels cooler ... So yes when water cooled the XZ won't cause this issue (ironically)
2. Buy a crystal cover which fits very tightly to the case .... (I use the one from Imak). This may help keep the cover in place when heating up and also transfers heat quite well.
Hope this helps reduce your frustration ....
On second thought we now have a remedy for when the battery dies

Play your favourite intensive game and the cover pops out on its own like magic and voila you can change it
(Every dark cloud has a silver lining .... )