Any bivy over your sleeping bag absolutely must be breathable. Otherwise condensation of the moisture coming from your body will soak your sleeping bag's insulation. Even WPB fabrics like Goretex used as a sleeping bag shell or a bivy will cause condensation inside when it's cold. The only way to use that SOL bivy would be as a vapor barrier, inside your sleeping bag and over only a base layer. You probably don't want to do that unless it's below freezing. As mentioned, though, it's a good emergency item for your winter day pack.
If you're doing anything but midsummer camping in the Cascades, you probably want a warmer bag. A couple of years ago, while I was on a trek up the Entiat Valley for my annual larch "fix" at the beginning of October, the temperature went down to 15*F each night. I was wearing all the insulating clothing I own, plus a vapor barrier suit (the latter under my insulation), inside a WM 20*F sleeping bag. I was warm enough on top, but my sleeping pad was insufficient so I was shivering underneath! I've seen freezing temperatures and small amounts of snow in the high alpine areas of the Cascades in early July and late August.
When I tried a silk liner, I got so tangled up in it (I toss and turn a lot) that it took almost 5 minutes to get out of my sleeping bag in the morning. When I ditched the liner after acouple of nights of that nonsense, I noticed no difference at all in the warmth of my sleeping bag. Your Mileage, of course, May Vary, but I've seen/heard the same complaints from many others. You're better off to wear more insulating clothing inside your sleeping bag and forget the liner.
With a full length zipper on the sleeping bag, or with a quilt, you can ventilate enough to be comfortable with a 20*F bag even on warm nights. There have been a few warm summer nights when I started the night on top of my 20* sleeping bag, but by 11 pm I was inside or under the bag and by 2-3 am I was at least partially closing the zipper. Nights cool off fast at 4,000-6,000 feet, or on the Olympic coast.