Last dive I did in my winter plumage was horrid. Way too many layers. Felt trussed up like a mummy. Layers inner to outer: light cotton vest; FE tech vest; Santi heated vest; FE vest; FE long sleeve rash vest; FE arctic expedition. Lost most of my dexterity. Only benefit was I was warm. Have tried a Santi BZ400 in the past, but it didn't fit my legs (I'd probably need a MTM one to get the knees in the right place). It also was very tight in my drysuit. So, Kwark undersuits look good and have a good reputation. What do you wear under them?