The bare drysuit suit, I am after says it is a close fit and that they are made to hold a 200 g undersuit.
Just a quick undersuit search and this is what Santi say -
The Santi Flex190 Mens Undersuit is a thinner and more flexible undersuit for your drysuit but it will still keep you warm in temperatures down to about 7°C. AquaBan™ technology wicks moisture away from your skin which immediately warms you and keeps you dry. 180g/m2 material traps an insulating layer of air around you and has a 0.96clo warmth rating.
If what Santi says is true, may be something like this suit would be fine for me, as I am not sure I would be diving in water much colder and if I do, I could always wear my lava core top/bottoms underneath, which are pretty thin.