@Inkedobiwan
Have you ever dove a 5 mm suit before? Dano can correct me if wrong but I don’t think the open vs closed or the suit itself that is the issue. It is the inherent buoyancy of neoprene. The more neoprene the more lead needed. The main difference, as the calculator you linked shows, is that the 2 pc suit does require a bit more lead because of the overlap of the 2 pieces. Just the nature of the beast.
The advantage of the Mako 2 pc is that most divers will actually get more thermal protection from less neoprene because of the open cell style which ultimately means less lead for the same warmth. This is what
@drk5036 was saying.
And yes, the 5 mm Mako will give you much more flexibility mm per mm. I sold my “stretch” 5 because I hated it and dove cold to avoid wearing it. I love my 5 mm Mako. There is a learning curve but after a few dives there is no going back for me.