Please help me extend the range of my 3-season sleep system.

I think if I had a Plasma and wanted to keep that I'd go with a 30 degree quilt. Clothes will get that down close to the Plasma so you don't have a gap in your range. Taking a 40 degree quilt all the way down to 15 is going to require a lot of clothes, probably more than you'd normally carry for say 20 degree mornings, and that ends up being less weight efficient than the 30 degree quilt and your normal clothes. Of course, another way to look at it is that if you get the 40 you could use it down to around 25 and if you expect colder than that you bring the Plasma. So either will work, it depends more on what is going to be the best setup for the temperatures you encounter the most.

And I'd say wide on the quilt.

i think paul gave you really good advice

You could get a quilt like the EE models that opens up,is read to 35-40*, weighs a pound, and over your bag would supplement your system another 10+*. If you are looking to go color than your 15* bag, though, I would probably skip the quilt. The Montbell ULSS #1 is the only mummy comfy enough for a side sleeper. No more straight jackets just sweet,sweet dreams. I'd also look at Zpacks bags.

Thanks for the input.

Regarding usual conditions, I backpack primarily in the PNW. Summer temps typically range 30-50+* and shoulder temps 20-40*. Winter temps may be 10-30*, with the lower limit of about 10* because if I'm expecting colder or significant exposure I bring a winter kit out of the scope of this topic. I typically need gear rated about 10* colder than actual temperature because of the wetness factor. Wind usually isn't much of a problem because I sleep in a mid and bivy any time I'm exposed.

You're absolutely right about the difference between clothes I would probably already be bringing vs. clothes I'd have to bring just to extend the quilt. I did a little numbers digging, and here's what I came up with.

EE Revelation 20* – 23 oz (L/W/850)
EE Revelation 30* – 20 oz (L/W/850)
EE Revelation 40* – 17 oz (L/W/850)

Stuff I always bring:
Pat R1 (L) – 12.8 oz. Probably my most worn item of clothing.
_AND_
light or mid baselayer bottoms ~ 5-7 oz
_AND_
Ghost Whisperer (L) – 7.3 oz
_OR_
Pat Nanopuff (L) – 12.6 oz (if I'm expecting continuous rain)

Stuff I would bring primarily to supplement sleep system:
MB Mirage (L) – 14.7 oz (in place of Ghost Whisperer or Nanopuff)
MB UL down pants (L) – 7.7 oz

From those numbers alone, it's pretty clear that the weight is better spent adding down to the quilt than bringing additional clothes, even if the MB down clothing is very warm for weight. In fact, I could go from a 40* quilt to a 20* quilt for the weight of the down pants.

As for the size, I know I have read quite a bit about people using quilts to extend the range of mummy bags. But I'm unclear on the fit. Does the quilt have to be fully open, or does the foot box of the bag have to fit into that of the quilt? Would a long/wide quilt be sufficient to cover a long mummy bag?

I can layer inside the Summerlite (puffy top and bottom as well as base layer) and that is how I do it.
For you the 5" wider (at the shoulders) Megalite could work well.

I'm not sure that I can effectively layer my ghost whisperer or nanopuff inside my summerlite. It SEEMS like my shoulders are tight enough to be compressing the down from the inside, but I have a good amount of room/loft in the chest area. If I'm sleeping on my side, as I usually do, the bag doesn't fit quite so snugly.

If you don't mind sharing, what is your shoulder girth? My own crude measurement around the widest part of my shoulders (about mid upper arm) is somewhere around 52" in a very relaxed posture in a t-shirt. With my lungs maximally expanded I get closer to 54". With a reported inside shoulder girth of 59 inches, I would expect the Summerlite to feel a little looser on me (room for a thin puffy). In comparison, the (reported) 62" shoulder girth in my Plasma 15 feels huge, almost too big without layering. The Megalite apparently has a shoulder girth of 64".