Like many of you, I have researched, bought and used a hockey sock (that's Canadian for a lot!) of mid-layers. Fleece, microfleece, primaloft puffy etc, etc. Many of these are great but, often overly warm. I still like and use my puffy jackets (Arcteryx Atom Lt and the new Kuiu Active) but, still not great for high output activity. I recently purchased an Arcteryx Naga hoody (full zip) as I was emulating an idea Evan made on using this type of fleece as a baselayer during high output activity. And this works super well. However, the winter in Eastern Canada has been pretty mild and the Naga hoody under my Windcheater is still overly warm, at times.
A year or so ago, on a whim through a discount place, I found an Arcteryx lightweight fleece known as the Stradium jacket. It is a very stretchy, air permeable lightweight fleece, with a wind and water resistant layer on the arms and shoulders. Like the Atom Lt jacket, the underside of the arms and side panels are quite porous, allowing heat and sweat to dissipate easily. This jacket is designed as a running or Nordic skiing mid-layer. I never used it as such until last night. With a polypro baselayer (HH Lifa), my Stradium jacket and Windcheater, I was comfortable and not wet at the end of some snowshoeing at 5 F. The jacket weighs next to nothing, is not restrictive (even under armour/LBE) and dries super quickly. I wish I had put it through it paces earlier!
I may have finally cracked the code (for myself anyway) in the ever fine balancing act of warmth to weight to heat displacement.
Hope this helps.