To me, the Seaskin Tech Base adds a tiny bit of warmth, but it is so minor that I would not ever bother wearing it over anything else. I only wear it for its wicking properties. So, it's against my skin or don't bother.
It is supposed to be snug. Wearing it over anything else, I would be concerned that it would be reducing the loft of the wool and reducing the warmth of the wool (by a tiny bit).
I have 2 sets of the Seaskin base and just ordered a 3rd. I like to start every day with a fresh set of undies. When I was in 10 days of training recently, I had the luxury of a washer/dryer where we were staying and did a load of laundry every other day, for fresh undies every day. If I didn't have that, then at least my undies get a full day+ of drying before I wear them again.
One thing I'll note about the Seaskin tech base is that it wicks very well, but it does take a while to dry. If it's pretty damp at the end of a day, it might not be fully dry the next morning, if it is just hung up to air dry overnight. Especially in a humid environment.
I can't recall why I wore it over top... I just remember doing so, lol. Maybe to see if it was better? Idk..
I do know that I went back and forth between the 2 sets with my older neo ds on multiple dives. At the end of the dive I liked they way the merino felt better hands down, whether wet or dry. The seaskin just felt like it wanted to "cling" to my skin whereas the merino felt like thin pajamas.