I recommend against the Weezle undies.
I own the Extreme Plus. It keeps you really, really warm. But compared to Thinsulate, it really really sucks. It's very puffy, and unless you have a significant amount of air inside the drysuit it's not warm at all. Furthermore, the outer material is made specifically to prevent air movement through it (wind resistant, after all), and it makes dumping *really* *really* difficult and sometimes almost impossible.
Get some Thinsulate undies. Actually, do you want to buy a size medium Weezle Extreme Plus with a bunch of knife holes over the left shoulder to aid air movement?