Mark,
In the testing we've done here, it does seem that Universal Clays burns at a lower flame temp than Unique. Of course we don't have equipment to quantitatively measure this here, but in our experience and test bear out your postulation.
Interesting too, I believe AA #5 to have an even lower flame temp than either Univeral Clays or Unique. A powder well worth it's place on the loading bench for mid range bullet weights in a broad spectrum of applications.
God Bless,
Marshall