I never had any problems with the UTH heating through the bottom of my T8s when I had them - I used Flexwatt and THG though, not Ultratherm.
For ambient temps, I was just fine without anything extra, but I had good control over the temperature of the room they were in. Others have found that they needed to add supplemental heating, like RHPs, to raise the ambient temps a few degrees. Like many things, the answer is "it depends".
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