Not sure, but I do know that you won't get a -15 synthetic bag very small. If you need warm and compact you'll need to go with down. Or wear extra layers at night and go with a different synthetic bag.

The weight won't change no matter what you do. With a compression sack, you will probably end up taking the stuff size from 9x16" down to about 8-9" x probably 12-14" or so. Or, like Ryan says, if you go to a down bag with a similar rating, you'll get much lighter and much smaller when compressed.