I have 3 of the 4 piece Whites system for under my Membrane Drysuit and have quite happily been in water 6° for 1 hour. (but i do like my Weezel Boots as well) I bought a bit at a time and found that I didnt need the jacket I was warm enough without.
Whites Glacier 4 Piece Undersuit System (undersuits) :: SDS Waterports for scuba diving equipment