I haven't tried bar mitts.
I've used
these when it is below 32F. I wore them the other day at 8F for about an hour and my hands were comfortable. The same manufacture has a lighter weight glove I use and work great when it is 32F and above.
The warmer glove has a bit of a wind barrier where the lighter glove does not have any wind protection, but at the temps I wear it I don't need a wind barrier.