I think you are on the edge of your lighter pads range, but the synthetic bag will help just a little to boost it. This is not scientific by any means, just from my past experience. If you can experience colder temps on your trip, or your Thermarest is so much more comfortable, I'd lean that way. It's hard to judge how much R-value the bottom of a compressed synthetic bag has. Here's a little test. Take a relatively flat package out of the refrigerator. Place it under your pad and sleeping bag. See how much cold is coming through both. Try it with your larger pad. Something from the refrigerator should be close to 5 deg. C.