Which winter road cycling shoes are rated for sub-15° F weather? Most of the road shoes I've found only seem to be rated to around 25° or so. Living in New England I'd like something colder. Does anyone know of road shoes rated for colder temps? 

(They'll be used for commuting and weekend road rides)

Update: I have new Neoprene booties (Pearl Izumi Elite Barrier Shoe Cover) but find that they're only comfortable down to about 25°F or so.
The vast majority of cycling shoes (road and mountain) are well ventilated and not suitable for cold weather riding.  Lake makes an excellent winter boot (I have several pairs).  45NRTH makes the Wolvhammer, which I haven't tried, but have heard good things about.  There isn't a market for cold weather "road" shoes since generally road bikes become unsafe/unuseable after there is ice/snow on the road.  

As far as getting the most out of the footwear you are using, I recommend NOT putting thick socks in your summer shoes.  Most people's summer riding shoes are close fitting, and adding extra thicker socks make keeping your feet warm harder since it compresses your blood vessels and restricts circulation.  If you can't stay warm with normal socks and booties / chem packs, you may need to look at purchasing a set of winter shoes with extra room for thicker socks.  My summer shoes are all 44s.  For winter, my shoes/boots are all 46s and 48s.  The 48s allow me room for two thick pairs of socks comfortably. 

If you are going to using chemical heater packs, I suggest sticking them to the top toe of your shoe under the bootie.  The won't compress your toes that way and will get better air circulation / stay warmer.