Can't comment on Wiggy's or anything current for mil-issue - never used the former & way out of the loop on the latter.
For some general gee-whiz stuff, no bag will keep you warm without being insulated from the ground - bag's insulation is compressed to 'bout nothing under sholders, hips, etc. Depending on what circumstances, you may be well served by a simple closed-cell foam pad. They roll up pretty tight & are lightweight. Maybe use a 1/2 length one to go under just shoulders to hips - clothing can be stuffed under the rest of your body.
GoreTex bivy bags are worth their weight - saves having to take a tent, stop the wind & prevents much convective heat loss - greatly improves bag's efficiency.
Decent bag, pad & bivy bag & you can be set for some pretty serious conditions.
Down is lighter for any given insulative value & compresses better than synthetics, but, the synthetics are catching up & perhaps, except for the most demanding, are just as "good" & are a bit less expensive.
Marmot is top-notched stuff & I use a 2lb5oz down bag (Swift - mummy ~$120) rated at 20F for 3-season CO use to 14K'+ An old Holubar (precusor to North Face) -20F down for winter.
Weight for my summer package is under 5lbs for pretty bullet-proof protection.
Big advantage of synthetics is you can squeeze out the water & it will dry much faster than down. A wet synthetic will keep you wamer than wet down, but it will suck anyway - keep it dry.
My one fast rule about bags is get one warmer than you'd expect to need - you can always get colder if you want.