I just finished a modular quilt setup which is basically a M50 shell you can tie in different layers of insulation.
Since I had some IX laying around, I made a liner out of a single layer.
Although some people have luck with it as a top quilt, it does not breath well enough IMO unless you never sweat.
IX weighs about 1.4oz per SY, so if you build a two layer quilt it weighs 2.8oz per SY and provides about the same insulation as 2.5 Apex just guessing.
IX is not cheap either.
For me it made sense as a liner because you do not have to wrap it in a protective shell like you would with climashield.
I tied it inside the M50 shell as-is and way too clammy.
What I ended up doing was to split some 2.5 oz climashield (+-1.25oz) and made a M50 1.25oz climashield round part of the foot box. A 1.25 oz climashield footbox end.
About 4" up from the round foot box end, on top I added a 8" x 14" 1.25 oz climashield panel covered in 5mm silk.
From there up to 6" from the neck, I cut a 1.5" slot and covered that with 5mm silk.
The bottom of the footbox is open down to the round end
Also Scalloped the edges.
The liner weighs 4oz and the shell 5 oz and rolls up about as big as a nalgene bottle.
So far so good but this is only a summer quilt or will be used as a booster on the inside face of the assembly combined with climashield liners.
Just guessing I would say it should be good to 55-60dF.