The (extra) weight you need will mostly depend on the volume of air that will be in your drysuit.
The volume of air is mostly dependend on the fitting of the drysuit for your bodyshape and the type of undergarnment.
Whereas the fitting is independent of the material, the type of undergarnment is likely to be dependent on the type of suit; i.e. the more insulating the suit, the less thicker undergarnment you need.
I am using a compressed 4mm neoprene drysuit and that works well for me with a leightweight undergarnment, even in the low 40F.
My first drysuit was a trilam and I had a need for having more air in the drysuit to stay warm.
Also, extra air can (and will) be an issue for buyoncy, especially for staying warm at stops.
Therefore, even so the material of the drysuit is a key factor (as mentioned, I prefer compressed neoprene for its insulating qualities) you should take the following factors overall in consideration:
(1) fitting of the drysuit for your bodysize
(2) material of the drysuit
(3) quality of the undergarnment. (Highquality undergarnment can have better insulating factors and meight therefore not need to be as thick)