G'day Mike,

I don't have an answer for you regarding a minimum temp but I do know that you will notice a big difference in pack performance if the pack is pre warmed. The local TP distributor here in Aus recommends warming the packs up prior to use not only for better performance but also pack longevity. You can damage the packs if they are too cold and under high discharge. A hot water bottle in an eski works great to warm up the packs.

Cheers
Jason.