I advise against the synmat 5. Unless they have changed the design, the 5 series does not have the outer tubes insulated. Yes, I know that is completely dumb. Regarding partial inflation and R value, it would depend on how much you partially inflate it and there is a possibility it would reduce the R value, but not enough to make a big difference. Experiment. Also be aware that there are two different tube designs; some have the tubes welded together at the tube intersections and some have a baffle at the welds. You definitely want to get the baffle type. A close examination of the tube weld junctions can tell you which is which.