I have a Cerium LT (one of the ultralight down jackets, and for what it is, this jacket is amazing. It feels like you're wearing nothing while keeping you warm). It's great with a regular hoodie. At about today's temperatures at 3 deg is when it's sort of reached its limit for being such a lightweight jacket. But there's no way this would withstand Toronto's weather.
I'm wondering if an Atom AR would fit under the Cerium LT. I went over to Valhalla Pure today hoping to try it on together, but they didn't have any AR's left in my size
Basically what I'm tying to do is have a double-lightweight-jacket combo, one synthetic and one down, so I can have the warmth of a winter jacket without the weight or bulk, and also the benefits of both synth and down. Also that gives me the option of wearing one over the other, or the other way around, or just 1 of the 2 jackets, depending on the weather and what I'm doing. I don't spend much time in heavy rain to need to get a waterproof shell (also I personally don't like how gore-tex and other waterproof shells feel).
I tried the Atom LT with my Cerium SL and it fit, but I don't know if I'm convinced it can withstand temps down to -20 when I'm on the east coast. Might pop by the Arcteryx shop tomorrow and try the AR.