this is a bit late. i can't personally answer your question. however, i think the Jacks R' Better website might give some insight.
i realize they are talking "down" here, and you mentioned synthetics. just see what you can figure out from their website and take some "reduction", so to speak, for synthetics vs. down - 15% less, 20% less, 30% less, 50% less??? i sure don't know.
[Caveat: they do mention "compression sack". so, not sure if you really want to compress and go that small. i've read that's bad, i.e. compression. but i gotta believe that these guys know what they are doing. however, having said that, personally, i wouldn't compress - err on the side of caution. it's up to you, i guess.]
go to their website's product page. scroll the page down and click on their "Quilt" links. while they don't seem to mention how much down (unless i missed it) is in each quilt, they do give the dimensions of both the quilt fluffed and lofted, and when stowed in its compression sack. so, you can calculate cu. in. of both and the compression ratio. do that for two or three of their quilts.
if you can find a website that gives similar figures for synthetic bags or quilts, that would give you a closer approx. ratio for your purposes. i, personally, can't remember a website that gives these numbers however.
not sure if it would be more accurate to use oz. of down and its fill power to calc. "stuffing"/compression, or the overall dimensions of the fully fluffed and lofted Quilt/bag/down-filled-object". perhaps, the second since the outer fabric is an integral part of the whole process/calculation.
since JrB only give dimensions of their quilts and the max. recommended compression sack, this is all you can use.
at any rate, you only want an approx. guide and not a precise to two decimal places number. plus outer fabrics will probably change the overall compression ratio, i guess?
just keep in mind that the ratios Jacks R Better obtains will differ for your gear depending upon type of insulation and type/weight of the outer fabric. but then, i believe you already know that. i only mention it for any newer readers who may not be as experienced as you seem to be.
so, without further ado, here's the link:
Jacks R Better - product webpage link - click me!
if anyone out there can add in dimensions (stuffed AND unstuffed) for their synthetics, that would help.
until then, perhaps, these number can give some starting point? just take a logical, conservative reduction from the down. again, don't know what that would be.
i'll also hunt around for a website that gives similar stuffed/unstuffed dimensions for synthetic bags and quilts. i'll post back if i find any.