I have a FF Grouse (the women's 30-degree bag.) I did consider Valandre's when I made my purchase though, to be honest, not all that seriously.
The big difference I see in your choices is the fill amounts. The Mirage only has 13oz of fill while the Swift has 18oz and the Kestral has 15.7oz. The Mirage is slightly narrower than the Kestral/Swift but not enough to compensate for the difference in fill amounts, imo. For comparison, the Western Mountaineering Ultralite is much narrower but still has 16oz of fill to be rated at 20-degrees.
I've found my FF's temperature rating to be spot-on. I sleep average and am "comfortable" at 30-degrees wearing midweight baselayers and wouldn't have any problems to 20-degrees by supplementing with additional clothes. I'm sure I could "survive" at much less.
Not saying that the Mirage isn't a good bag, just that you should be realistic about its temperature rating. It's comfort rating is 35-degrees and I would expect it to be about 5-deg colder than the Kestral and 15-deg colder than the Swift. Imo, that's a pretty mighty price tag for what is effectively a 30-degree bag with a 3/4 zipper.