FWIW, if you are willing to drop $350 on this purchase, it seems obvious that money is not your main consideration in seeking a synthetic bag instead of a down-filled bag.

I do not know what reasons are behind your choice, but I will only point out that, if you wish a sleeping bag that is "as light as possible yet still very efficient", then down is the obvious way to go. Down will be lighter, warmer and (with proper care) it last much longer than a synthetic bag.

If, by chance, your reason for choosing synthetics is a fear of getting your bag wet, then consider this: a wet synthetic bag will not keep you very warm, either. Most especially in near or below freezing temperatures. The superiority of synthetics when wet is a marketing myth, not backed up by research or in the field experience.

If, however, your reasons have more to do with philosophy, such as veganism, or strong convictions that down-gathering is cruel, then clearly a synthetic bag will avoid these particular objections.

Good luck with your purchase.