Sooo thinking more and more about making myself a quilt (and less and less about my schoolwork...)

I am thinking of making something that could be versatile: a rectangle with drawstrings on both ends and maybe omnitape up the side so I could use it as a sleeping bag/quilt or cinch it up as an underquilt for a hammock...

So... looking at thru-hiker's climashield xp insulation: two weights are listed, 2.5 oz to 40 degrees, and 5 oz. to 20 degrees. Somehow, I'm just not trusting their temperature ratings

Can anybody who's used this give me advice?

How warm will the 2.5 oz really be as a top quilt? As an underquilt?
And the 5 oz? As a top quilt? As an underquilt?

(And if as an underquilt, let's assume I'm using a 40 degree bag as a top quilt)

I'm a cold sleeper : o )