How warm can you take your 20F quilt?

Can you list the specs for each, arc alpinist and ultra? The ultra specs may be close, but I doubt they can be the same since the ultra uses heavier fabric at least at the foot. Also, I wonder what the ultra fill weight is.

Arc Alpinist (size long):
Foot/Hip/Shoulder Width: 38", 45", 55"
Fill weight: 12oz
Loft: 2.5"
Differential Cut: Not stock (I think you have to ask for it)
Total Weight: 22 ounces

Golite Ultra 20 (size long):
Foot/Hip/Shoulder Width: Exactly 2" more width across the board
Fill weight: No spec available; I've been meaning to call Golite
Loft: 2.5" (stated loft – my rough guess is MAYBE 2.5" at peaks, lower at baffles)
Differential Cut: Yes (per Golite's site)
Total weight: 21.2 (measured)

Both have two straps.

It seems from the specs that the warmth is probably somewhere between the Alpinist and either the Ghost or Specialist. Comparing the Golite with the Specialist or Alpinist should be as close to apples-apples as you can get considering they are extremely similar in design.

I use my Arc Alpinist on almost every trip. 50F is fine since it's easy to open up all or part of it, stick out one or both feet, or kick it to the bottom of your shelter in unexpectedly warm temps. Personally, I enjoy owning one quilt that can take me comfortably through all 3 seasons. Something lighter would be fun to play with, but for me it'd be a specialist item rather than a mainstay.

I'd guess/bet the golite ultra fill weight at 10 oz.

I will point out the obvious here…your Ultra is a quilt. If it gets too warm, just pull it down or take it off altogether. I've used a 20 degree bag in temps that ended up not dipping below 60 degrees all night. The bag was unzipped, I adjusted coverage accordingly.

You're really talking about a narrow temperature range here. If its going to be below 40 degrees, you're probably going to want to take the Ultra for added insurance and if its above say, 55 degrees then a bivy or a silk/fleece bag liner would fit the bill for the same/less weight and a tiny fraction of the cost. In my observation, the real question is is the expense of such a specialized piece of gear warranted to cover such a narrow band of functionality? Of course, you could use the Arc Edge in temps below what you're stating but then you're cutting into the temp range that the Ultra was designed for and (for me at least) that smacks of getting less functionality for the investment you've already made.

The only reason I see to buy another quilt would be to shave off a few ounces, which for me would be more of a mathematical exercise than a real gain. Of course, maybe your budget allows you to spend that kind of cash to drop a few ounces. I've considered a quilt for specific summer usage but I'd probably go with a cheaper alternative than the Arc Edge. I think you'd be better served using the bivy and some additional clothing.

Fill weight is mentioned on the practical backpacking podcast.
I do not remember the specs, but I do remember they discussed the fill weight.

Fill weight 10 oz in the regular, 9 in short, 11 in long.

60/52/39" at shoulder/hips/footbox

Probably more of a 25-30 degree bag.

I have taken my 15 Degree quilt up to the 90 Degree Range. I just kick off the feet lay on top of it. Stand it up to block a draft.

However as far as cost goes I have a Lafuma 600 that cost about $70 and weighs in at 20 ounces. I have used it with and without my bivy bag. If cost is an issue this bag is great.

I had my Arc Specialist down to at least 16 degrees. That was the temp when I got out of bed when the sun was hitting my tent. I wore silkweight top and bottom, soft shell pants, R1 hoody, midweight socks, and a beanie. My head got a little cold so I covered my head with my down jacket. I slept on a Torsolite with the GG long Thinlight underneath. My feet got cold early in the AM but I placed the frame pad from my pack under my feet and I was OK.

I slept very comfortably before and after the cold feet incident.

This got me thinking. I'm getting ready to buy the golite ultra 20 as my main bag. I've currently got a Montbell Down Hugger that I'm using during the summer and a mountain hardwear conness 32 as my 3 season bag. If I bought the ultra 20, I could use it for all seasons and just supplement it with clothing during the winter. The ultra 20 weighs just 4 or 5 ounces more than the down hugger. I've considered just selling the down hugger and using the ultra 20 during the summer and using the quilt. I've also considered selling my montbell alpine light jacket to replace it with the alpine light parka, since the ultra 20 does not have a hood. Any thoughts on whether this would be worth while? any advice/comments appreciated.