Those numbers are about right. But, you need to size the bags so that the down can fully loft. Putting the synthetic next to your skin and using an oversized down bag is the best approach for efficiency of the combo, but you'll lose efficiency when using just the too-big down bag.
Better yet, use a synthetic bag and down quilt. This way you have synthetic underneath you, where it does more good than down (which just flattens), and the down quilt will easily spread wide to cover the synthetic bag.
A MH Ultralamina 45 is 22oz.
A nice down quilt rated to 20-25 is similar weight, probably a touch more.
Both are kinda pricey.
However, I don't like this approach much. I think you often end up with a heavy, less than optimal sleep system, just to get down to 5º once in a while. There's nothing nearly as efficient as a nice down bag once you get down below about 25º, so why try anything else?
A better system is to choose two bags that each cover a very wide temp range on their own. Each temp range has a different style of bag that works best, so optimizing the design can net you big weight (and cost) savings, and still get you a very good bag for any conditions:
A nice 10-20 degree bag can be pushed as cold as you need in the PNW with extra clothes, an excellent ground pad, and proper shelter choice. That bag will weigh 2lb. I'd choose the WM Versalite, or the Ultralite (Alpinlite if you're big) if you spend a lot of time in the 25-35 degree temp range. An Ultralite with an R4+ pad, warm jacket with hood, and bivy/tarp or good tent will easily work down to 0º. And, unzipped and sleeping nekkid, be very comfy to 40º+.
Of course, you can get a similar bag from MontBell (UL #1), Marmot (Helium), Rab (Neutrino 400), Valandre (Mirage), and Feathered Friends (Swallow). All are excellent, I just like WM best.
For warmer temps, a synthetic quilt has very little weight disadvantage over down due to simpler construction and inherent efficiency of batting-style materials in thin layers. For 20oz or less you can have a 3.7oz Climashield Combat quilt with top and bottom drawstrings and zippered footbox (I like Tim's design for versatility, and he'll make one for $170). That will be good from 35 (right where the down bag gets stuffy) to near-hot conditions.
Some people like a 5oz quilt for versatility. I find that I really want a proper hood and collar below about 30º, so 3.7 is about right for me. You may wish to push the quilt a touch colder than I.
Does it get any more flexible than that? You'll use the quilt most of the time, wearing out a $170 item every few years and usually carrying a comfy 1.25lb for your sleeping bag. And nothing stays dry better or dries out faster than a single-layer Climashield quilt with seamless ripstop shell. Perfect for those damp Cascades 3-season trips.
The down bag is for chilly nights and will last for decades of moderate use. Being 2lb or so, you won't hesitate to grab it when you see the forecast dipping down below freezing. And, in the winter you'll feel confident bedding down in near-zero temps as long as you brought the ProLite Plus or extra foam pad.
This is slightly different than where I've ended up, living in the NE and focusing on winter alpinism. But, it's taking the same philosophy that's worked really well for me and applying it to your situation.
