Well, as I posted elsewhere, I broke down and bought a used DUI 400g Thinsulate jumpsuit, and I now have four dives on it.
All I can say is that I wish I had been willing to do this a lot earlier. Out of four dives, I've been cold once, and that was at the end of 67 minutes in 48 degree water, with nothing more vigorous than a modified frog kick, and a lot of hovering and admiring critters. And I wasn't shivering, just felt cold. Less than an hour later, I was ready to go back in the water and do another 45 minute dive (with a scooter, so even less exertion) and come out warm.
I'm impressed.