I recently asked flashlightlens.com a similar question:
I'm making a Maglite using a WA1185 hi-intensity bulb. Can the UCL handle the heat, or should I get the Borofloat instead? Also, is the Borofloat less "clear" than the UCL?
His answer was:
I've only heard of a few instances where a UCL cracked, but Borofloat is a pretty safe bet for that light. By going with the UCL, you'd gain about 4% over the Borofloat.
So, from what I understand, if you're running a hot bulb in your Mag, go with Borofloat. If your Mag is modded with an LED or just has the stock bulb, go with UCL.
-gvmelbrty