Tin Man, if you feel purchasing the FF bag is a waste of money, so be it. That's fine. everyone has a right to spend their own money how they see fit. I think. But don't say the FF w/Epic or Nanosphere shell and Campmor bags mentioned are going to perform the same under the same conditions in which the OP plans on using his sleeping bag and bivy set-up. They might not! The difference in shell options and performance was why he was considering the FF bag.
We might also consider that the OP may wish to simply cowboy, without a bivy, leaving it home when conditions dictate, but still wants a sleeping bag that has a more protective shell just in case. The OP may also do climbing or summit bids in dry cold weather and still desires to protect the loft of his down bag without also hauling a bivy.
Not everyone hikes, or camps, the same!
This thread isn't about bivies or bivy speeches, but since you brought it up, NONE of the sleeping bags I use with my bivies require any kind of special specifications because they are used w/ my bivies. I use the same sleeping bag whether I'm in my tent(s), hammock, under a tarp, cowboying, or in a bivy.
Tin Man, you bring up a good pt.though. If using a bivy, with its perceived limitations, requires having some kind of special expensive sleeping bag then that may not be the best shelter option for everyone. In that sense you are absolutely right. There are alternatives that CAN BE lighter, more comfortable, and possibly less expensive!