This is in support of your fleece quest:
I don't know if it is psychological, but I feel warmer when I have a combination of layers on of different materials.
I usually use high loft goose down for my main insulating mid layers because I don't think anything else comes close in terms of warmth to weight and warmth to packed down size. But when I put a thin fleece under the down it seems to really add something different and when I put a windproof shirt over the down it really helps.
Up top, if I have a mid weight base layer, thin fleece, down vest, down jacket, windshirt, and then a thin shell; I can sit still in very cold weather and I'm okay (I add 3 or 4 Hot Hands body warmers on the base layer).
I only use light and midweight base layers, but this thread has partly convinced me to buy this and pack it in as a fleece shirt.
As my packed in base layer in a larger size than I would buy for a true base layer. I use the same redhead series for my thin/medium base layers (but without a zip). They are very good for the price and are just a polyester fleece that has been treated with silver so that it doesn't stink.