I've used r-value of about 3 plenty of times in those temps. My x therm is only 5.7 and I've used it well below zero.
You can take the sum of r values that are layered. Sometimes I use an r1.3 air pad and a ridgerest (2.3?) Under it because I like it for glassing and it protects my air pad. I've done that plenty of times well below freezing.
Those klymit pads are great. I have the uninsulated and two of the guys I hunt with have the r4.4 pads. I like them just as much as the TAR pads.
I'd be more concerned about your bag than your pad in the case you described. If your bag lists a lower limit EN rating of 22°F, the comfort rating is probably like 35. Your money would probably be better spent there and just get a $10-$20 closed cell mat to supplement when you're pushing the limits.