It is the 23' cable and without regulation the temps on the tile hit 95. I did have a bit of trouble with one rheostat(lamp dimmer from home depot) keeping temps regulated so I went with a thermostat(herpstat which was expensive unfortunately) and everything is fine now.
Although with the duplicate setup on the other side, the second dimmer I bought was fine. So I may have had a bad dimmer on the one side. Personally, I really think the 9 year old was messing with the dimmer during the day but he won't admit it! He blamed it on the cats which is entirely possible as well.
So in short, the thermostat works great keeping it at 86 even on the left viv and the dimmer for the second cable on the right side viv is doing well(I taped it in place) ranging from 84 to 87 throughout the day and night.
Those tiles are 18" by 18" and the 23' cable covers roughly 12" by 16" at 1" spacing give or take a half inch here or there. For reference, my vivs are 36" x 20" so exactly 1/3 is covered lengthwise and 16 out of 20" in depth are covered for heat. The whole tile is not heated. Just the 12" by 16" section.
It probably wouldn't have been 95 if I had followed the 1.25" instructions but then it would have gone past 12" and I wanted to stick with the 1/3 of the viv rule. Remember that of the 23' of cable, only 17' is actual heat. There is 6' of dead cable.
I've had no issues with the tile or sealant and in fact, both snakes in each viv seem to love the 2" space between the tile edge and the front wall of the viv. It stays around 78 degrees there where as the cold side is 72 and the hot spot is 86ish.
If you use bump feet, it will be a pretty big space between the tile and the wood so there would be more to seal and it might not stay properly. Plus, if you seal it, it would defeat the purpose of the bump feet because the heat couldn't escape anyway because of the sealant. I figure if the structural wood in my house attic can stand the 150 degrees it is up there in the summer in Texas, the polyurethane sealed viv wood will be fine in the high 80s.
I don't know about roofing tile but I would think it would be more resistant to heat since it is for roofing. But I really have no idea. I know the cheap floor tile works though.
I think I covered all of what you asked but let me know if you have any other questions. Good luck!