I took a base-camping winter trip with a girlfriend once when we were expecting temps well below 0*F. She got my subzero bag, I used my 0*F bag. Temps bottomed out around -10 to -15*F that night, & I slept plenty warm. Although my bag was "inadequate," my sleeping pad arrangement was "overkill." I used an Exped Downmat 9... and a Ridge Rest, combined total around R-10. No cold-seep through that pad combo!
My experience has been that it's never a bad idea to have more insulation underneath you. What you might investigate, though, is the most weight-efficient way to do that. In other words, a downmat 9 is R-8 vs R-6 of the downmat 7, and the downmat 9 only weighs 4 ounces more. To achieve the same R-8 w/a downmat 7, you'd have to add a ~14oz ccf pad (ie, std ridge rest).