Surely, if you wear mesh under it, it's no longer a base layer.....
In my experience two thin close fitting baselayers is the way to go when it gets very cold.
Moreover, in my experience none of the modern, supposedly high wicking, synthetic baselayers offered by the likes of Helly Hansen, Craft, Castelli, Howies etc. are as good as the version of Helly Hansen Lifa sold 30 years ago. The older version of Lifa looked scruffy and was responsible for the 'Smelly Helly' nickname, but was very effective. The newer version might look better, but does not perform as well (in the same way that fleece supplanted fibre pile, despite fibre pile's superior performance on every count except looks/fashion). I think the reason for the superior performance of old Lifa was its rib knit and the 'bobbly' surface of the fabric, which gave it its scruffy appearance but also helped the wicking and insulation performance, unlike the very smooth surface finish of modern Lifa tops and similar products from other manufacturers like Craft etc.
The combination of Brynje mesh with an outer baselayer (merino or whatever), is the only product I have found so far to compare with wearing two old Lifa baselayers.