People are different, so is their tolerance to cold. But I'd say you'll probably keep decently warm. I have a 200 gsm Scubapro undersuit, and I use it with a thin base layer from 10-12C down to ~6. If the water gets colder, I layer with a slightly thicker wool t-shirt or two and that keeps me nice and warm down to 3-4C. A not too thick polarfleece shirt is also an option, just make sure it's got a crew neck
I probably wouldn't use the Lavacore. Granted, I have no personal experience with those products, but AFAIK they're designed for use under a wetsuit, not as a dry layer. Insulating layers developed for topside use are probably better suited (heh!) for keeping you warm while you're (reasonably) dry.