I think you'd be pushing your luck. My JRB sewn-thru Shenandoah quilt, which is accurately rated to 40-45F degrees, has 6oz of down, and weighs 15.5oz. A Montbell combo like you proposed has 4.5oz of down and weighs 15.8 oz in size medium. And then when you add in heavy socks and gloves, if you can't keep your hands up inside the sleeves, there's barely any weight savings over your Golite Ultra 20 + the jacket, since the down pants and heavy socks (3-4 oz?) aren't exactly multi-use in those temps.
I don't think the money savings would even be that great, considering the cost of UL Down pants(though I guess you could use the down pants for snow camping too) vs the cost of a lightweight summer quilt like the JRB model or the BPL synthetic quilts.
Metabolism drops a lot when you're sleeping, so what you're comfortable in awake, even at rest, may not be enough when you're sleeping. You could open your bedroom windows at night and try sleeping in the combo you've proposed to get an idea.
You can also consult this chart and this one for more ideas about what is okay for what temps.