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.