The Arc'Teryx Sirrus uses a 70 denier, 4.9oz/yd2 (168g/m2), 3-layer Gore-Tex XCR fabric in the high-wear yoke and shoulder areas, and lighter Gore-Tex PacLite III fabric in the rest of the jacket. Even so, the Sirrus weighs only 12.9 oz (366 g), typically about 1 oz (28 g) heavier than the lightest Gore-Tex jackets available today (e.g., GoLite Phantom and Mont-Bell Torrent Flier).
The jacket’s real strength is its durability:weight ratio. It offers a roomy, well-fitting hood equally suited for backpacking and sheltering a small climbing helmet. With a short (harness-friendly) hem, the Arc'Teryx Sirrus SL will find appeal among alpine climbers.
The Arc'Teryx Sirrus SL offers few features: front and rear drawcord adjustments and a stiffened brim on the hood, pit zips, one non-vented napoleon pocket with a water resistant zipper, and a drawcord hem. Lack of other pockets, combined with its trim fit and short cut save significant weight by minimizing fabric used. Further weight is saved through laminated seams with small seam allowances, a narrow storm flap backing the front zipper, narrow die cut hook-and-loop cuff closures, miniature cord locks and fine diameter drawcord. Some weight is gained in the articulated collar design - which allows the hood to function independently of the collar - a nice comfort feature when you have the jacket zipped up and the hood is down.
The $300 price tag isn't going to win any performance:price contests, especially when compared to more functional (feature-rich), lighter, and less expensive competition from the likes of GoLite and Mont-Bell.