It looks like the Hollis undergarment is pretty thin with 100g thinsulate between some fleece (inner layer) and a shell of nylon.
That said, I wouldn't dive that in water as cold as you describe. 100g of thinsulate is very thin (I dive a 200g jumpsuit from time to time in 68-72 degree water, I get cold after about 2 hours). Plus, I question the ability of that undergarment to keep you warm if the suit floods.
While some of the Hollis products are good, for the type of diving you're thinkin' about, I'd pass on this undergarment. You would probably be better served looking into a Santi or DUI thinsulate based undergarment.