In reply to BnB:
You're right about the danger of reducing the weight of fleece, if that's all you do. I should have stressed that this should be done when adding the fifth, over layer to the system, to cope with stops and compensate for the reduction in static insulation.
Since TNTTNT asked about walking, arm movement isn't that important, but much more worthy of thought for climbing...
And it seems that TNTTNT has an unusual physiology, or hasn't adapted from equatorial mode, since he (?) appears to need a lot of insulation, even when warm. I'm currently skiing (at anywhere between 70-100km/h according to the GPS) in about -13C, wearing a variation of a four-layer system, and not getting uncomfortably cold. I've had to use a light down overlayer for a short period after being exposed to very strong, cold winds for a period.
The new insulations are interesting, but I'm sceptical of miraculous claims... Anything that allows airflow through the fabric is a double-edged sword; allows the fug to escape (good), but the heat also escapes (can be good, can be bad). As with soft shells, it's a balancing act between protection and permeability. Control of airflow through the system is key.
To the OP; I wouldn't recommend using multiple merino base layers, as they absorb and hold sweat. I'd recommend a thin base layer to wick sweat away from the skin, and encourage it to evaporate. The mid layer fleece allows the evaporated sweat to get to the shell layer, where it will hopefully escape, whilst retaining a layer of warm air about you, and not absorbing too much sweat. Sweat can condense in the fleece, but the open structure of most fleeces means that they don't wick well, so the sweat doesn't wick back to the body as much as it does with a base layer fabric.
Using an overlayer stops you needing to take all your clothes off to add a layer, so you don't lose all that trapped heat, and don't expose your core to the elements.