I carry a Wiggys bag, I have 3 sets of the FTRSS, a mummy superlight for me, and the rectangular hooded (hunters) superlights for the girls, hence the purple request. They are big and a little heavy (as far as bags go), but with adequate pack space, it is worth it. My next bag is going a Kifaru MOB set, probably the same temps as the Wiggys. I own a Kifaru Woobie and I am sure I could go down to the mid 40s with it just as is, I am sure their bags will give me the same positive results, they are lighter and compress NICELY. I like the poncho and bivy setups for summer camps. Always take a hammock with me during summer trexs out here, hence the Woobie for sleeping since crawling into a bag in a hammock is a PITA. Placed a order for the Doobie and another pack last week. Got a estimated ship of 8-31, so a wait will be expected from them directly.
On ARFcom alot of people are praising the knockoff MSS found here
http://www.sportsmansguide.com/net/cb/mummy-sleeping-system-by-guide-gear.aspx?a=333009 I do not have it so I can not comment.
One thing I found annoying about most compression sacks in the ball symdrome you get. Unless its attached outside the pack, it is a PITA as well. I have started using "granite ware tubular stuff sacks" I can fit the overbag into a med, and the base bag into large (Wiggys FTRSS superlight Xlong Xwide). Either of which will now fit any bag I have, and both will fit into 2.
For MSS, the cheapest I have found is ebay or IMS surplus. Ebay as you know can be a gamble. I have purchased a pair of Gortex Bivys from "user name: gruntgear" Both have been unopened still have labels attached for under $100 each (75 and 65).
One thing I found annoying about the MSS when in the Marines was either A) prior Marine destroying them, boots woren while side and tear up the foot area, or B) poor loft from the prior Marine storing them compressed for a few years. I have no commercially avilable lightweight sleepgear. But trailspace may help you search out what you need in that area.
EDIT to add photos.
Wiggys FTRSS the one packaged in the middle is identical to the purple one. The mummy bags pack down a good deal smaller than the hunter series. The bags are mated in the photo.
Here is the base bag in gortex bivy, on a multicam poncho that I posted on ARFcom and ZS to show a few members the size of the poncho with the tail undone (great poncho/ tarp) FYI.
Kifaru Woobie... in the attached stuff sack, then compressing it by hand (the bags of theirs does the same thing) due to the design of the lose fill bag, it can help clear out voids in the pack and silence stuff from banging together, you can purchase a stuff sack for it separately if needed, good enough to use a bag to the mid 40s/ low 50s IMHO.