I'm not sure if anyone wants ALL the information but it was at least interesting to me :-) I'm going to go with the Poly-Fil Extra Loft, it's not exactly the same... but the only close contender is Soft and Bright I'm pretty sure I wouldn't be happy with that since if I just was going by the data and also that I've already based this section... it really would be a good choice.
While there might be better battings out there, I've already taken this a bit far!
Battings:
Soft and Bright from: LQS label: sew 10" apart, label talks about machine sewing, nothing on hand sewing 4oz.
Poly-fil Project Fleece from: Joanns. label: hand or machine, sew 4" apart
Mountain Mist Quilt Light from: Amazon, label: sew 5" apart
Poly-fil Extra-Loft from: Joanns. label: hand or machine, sew 4" apart
Poly-fil low loft from: Amazon label: hand or machine, does not list closeness on bag but website says 2"-4". 2oz
the original unknown.
(I started a test with a double layer of the thinnest, but it was so thick and difficult to quilt that I didn't keep going)
Process: Cut 7" squares of a typical weight/quality of top fabric, thin muslin back, and one of each batting. Quilted at least half of a simple snowflake pattern on each. They were marked on the back so all tests were blind.
Stiffness: scrunch up the square in the center
Thickness: holding both behind my back, try to tell which is thicker.
Quilting: I wasn't consistent with needle use (I think my earlier complaint about Soft and Bright was a larger needle)... and I can't really tell the difference for most. In the run-off, Soft and Bright was slightly harder to quilt than Poly-Fil Extra Loft in some directions.
Stitch definition
Stiffness: most stiff -> least stiff
Soft and Bright
Poly-fil Project Fleece
Original
Poly-fil Extra-Loft
Mountain Mist Quilt Light
Poly-fil Low Loft
Thickness (thickest -> thinnest)
Original
Poly-fil Project Fleece tied with Soft and Bright
Poly-fil Extra-Loft
Mountain Mist Quilt Light
Poly-fil Low Loft
Stitch Definition.... they all looked fairly close and well within any range, given things tend to flatten out over time I wasn't worrying about this very much... but on close look can be ranked.
most definition: Mountain Mist Quilt Light, Poly-fil Extra-Loft
medium definition: Original, Soft and Bright
least definition: Poly-fil Low Loft, Poly-fil Project Fleece