I do agree with you on the snakes temp mattering but I personally don't see how a UTH could heat the ambient above a couple degrees at most. I mean if the snake room is 75F, the best you could hope for with a UTH would maybe be 77F. This is assuming the cage is a small cage that is well insulated and you aren't using a fluorescent light which will heat the enclosure a couple more degrees. I don't really use UTH so maybe there is something I am missing as I only use RHPs in PVC cages and CHEs in glass cages.
From what I have seen from people who use UTHs exclusively like in racks is they have an oil heater in the reptile room controlling the overall ambient temps and the UTH is just creating the 90F hot spot on the floor for digestion.