IMHO, the above comments are right on. The Reflectix will need to fit you closely, at least around the edges, so the trapped warm air can't escape when wind pushes on it. (In still air, voids between your body and the Reflectix won't be too much problem if all of the edges seal well to your body, but a wind will continually pump the warm air out of those voids.) This is one advantage of a thick outer quilt: some areas will be thinner and others full-loft, but it will conform fairly well to your body shape. I found at least a 20F difference in comfort range just by adjusting my under-quilt to fit my body more closely.
An advantage of Reflectix is that it is easy to shape (cut out a thin wedge then tape the edges together), but I'm not sure you could shape it close enough to a body in a hammock, especially if you change position. You could sew velcro dots all over the underside of your hammock, glue or sew velcro dots onto the Reflectix, then stick it tight; it might hold the Reflectix close to the hammock, or maybe the dots would all tear open when you got in (it's just an untried idea.)
Using the Hennessy Hammock SuperShelter underpad/undercover combination (the underpad is open-cell foam and the undercover is silnylon), I found condensation inside the undercover every morning. So Reflectix is sure to have condensation if the weather conditions permit it to form. However, as long as it didn't touch you, it shouldn't be a problem. But my guess is that when you sit up in the morning you're going to get a wet butt.
I use a JacksRBetter Nest down quilt (same as Eric's No Sniveller except the slit is cut to match a Hennessy Hammock) as my cold-weather under-quilt. This quilt, with or without the JRB Weather Shield bottom, has never accumulated condensation; it weighs 20 oz. and keeps me warm well below 0F. The Hennessy Hammock SuperShelter Explorer-size underpad/undercover weighs 20 oz (claimed 14 oz) and keeps me warm to below freezing. A body-sized GossamerGear ThinLight 1/8" sleeping pad (2.5 oz) or an Axium Jumbo "Blockade" autoshade (similar to Reflectix; 1/4" thick; 4.4 oz; it tore) inside the hammock keeps me warm down to about +40F. Reflectix equal to the surface area of the JRB Nest would weigh about 15 oz., and I would be surprised if it would keep me warm much below +40F. There could be a 40F or greater difference in warmth for the extra 5 oz of a JRB down underquilt compared to a Reflectix underquilt. I would certainly expect that a thin Pertex/Primaloft underquilt would be warmer, lighter, more compressible, less condensing and just as durable as a Reflectix underquilt. A Reflectix underquilt might be a bit cheaper than one made from Pertex/Primaloft, but cheap Walmart nylon and batting would probably be about as cheap and still work better than Reflectix. As long as your tarp sufficiently covers your hammock, weather shouldn't be much of an issue for your bottom quilt. Mine hasn't become wet from rain or driven snow.
YMMV. The above is based on my experience with down and SuperShelter underquilts, and various sleeping pads (including something like Reflectix) in the hammock. I've never used Reflectix as an underquilt.