From this recent thread on Exped mats:
Ken had some excellent posts, including the following EN standards:
"To remain thermally neutral at the following temps the corresponding r values must be used with a proper EN rated bag and a light baselayer.
Similar stats have been posted in the past, IIRC, by Mr. Nisley.
My experience has been pretty equivalent to the above numbers, including plenty of time in the -20F to 10F range.
A Neo plus a Thinlight is something like R-3 to 3.5, ie half of the thermoneutral value for 0F. In my experience, I've found that R-8 is necessary around -20F to 10F (R6 acceptable at 10+), & a Downmat 9 works great (R-8). Have also used a standard Thermarest w/a ridge rest quite happily (~R-6). If the pad isn't warm enough, the rest of your system (including you) has to work harder to stay warm. Not saying that it isn't possible to use a lower R-value pad in such temps, but that it is unusual for that lower R-value to be sufficient for most people.
Also note that when sleeping bags are rated at a given temp, they assume an adequate amount of insulation underneath them, as above. In other words, if you have a negative 20F bag on a thinlight in +20F weather, you'll probably be cold.
Just to clarify from the OP, that no insulating clothing bit was specifically for the sleep system, right, but you would have insulating garments in your pack?
One other note, on the large Neo... the wider pad w/a narrower bag can result in some additional airflow/heat loss. I talked to someone at Cascade about this & they agreed... better in cold to have the sleeping bag pretty much covering the pad.