Most manufacturers target two weight points for an insulated ultralight jacket: around 8 to 12 oz for a lightweight jacket, and around 25+ oz for a belay-style parka. There are only a few jackets between those weights, such as the Patagonia Micro Puff Jacket at 16.5 oz with 2.7 oz/yd2 Polarguard Delta, and Wild Things hooded EP Jacket with Primaloft insulation.
OR jumps into the fray with the Zero, a 20-oz jacket (size large - a medium could be around 18 oz to 19 oz). The Zero uses 4.0 oz/yd2 Primaloft One insulation in the torso, 3.0 oz/yd2 in the sleeves, and 1.8 oz/yd2 in the hood - intelligently placing the most warmth at the torso. The light hood insulation assumes that one is already wearing a hat, balaclava, or helmet. OR knows about climbing. The hood is large enough to cover a climbing helmet.
The Zero weighs less than a 25+ oz belay parka but may come close to the same warmth. Best of all, the jacket retails for a reasonable $169.
"Outdoor Research Zero Jacket - 20 oz Hooded Jacket with 4.0 oz Insulation First Look (Outdoor Retailer Winter Market 2005)," by Alan Dixon. BackpackingLight.com (ISSN 1537-0364). http://backpackinglight.com/cgi-bin/backpackinglight/outdoor_research_zero_outdoor_retailer_winter_market_2005.html, 2005-01-30 03:00:00-07.