Actually the Kappa was my shortlisted preference, your reviews and my research backs your positive experience. The issue is it looks on paper too warm in the around-freezing temperatures, 133g core and 100g arms. I've hovvered over the buy button a number of times.
It does though for a synthetic tick all the boxes of good warmth/weight and effectively waterproof via design (few seams), fabric (HS2) and the PL1. Seems few use PL1 which is better on warmth/weight and holding insulation when wet.
I was thinking though that a single warm item is easier to vent, less hassle as undoing the zip will get cold air right inside, so the too-warm issues might be mute? I knowdown jackets being so light and airy can be opened up quite a bit, but still the issue of the backpack and the arms still trapping a lot of heat.
The Prism, my current fave, it can be boosted by layering, but the easiest layering which is a gilet, works fine as the temperatures just get tool-cool, but fails to work at lower temps as the arms are cold, so have to do the traditional mid-layer approach. That's what I did last winter.
So that is really my dilemma, basically layering which is more flexible over wider range of condtions vs the Kappa which risks too-warm but simpler.
If the Kappa was say 100g core and 75g arms, I'd have bought it already.
What I might have to do is simply buy a Kappa, look after it, let the next winter pass and then keep/sell. I'm probably at the limits of what I can do in my head. The Kapa looks very smart too 
James - just above freezing, when active, how is the insulation? So I know that temperature and a Prism 40g plus 100g gilet is about-right so that's roughly 133g core and 40g arms and I reckon the Kappa too-warm, but it comes into its element when still around-freezing and obviously active when lower temps.
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