Before you ask me if I'm out of my mind :) , let me explain...
In Maui (13 dives over 4 days) with 77-79deg F water, I wore a 3mm farmer john (mine) under a 3mm fullsuit (rented) - extreme comfort, no chill, no overheating, dives easily extending over 1hr length...
SoCal 54deg water...ok...