If you are moderately active, as opposed to sitting for a long period, the base layer and shell you specified, combined with something like the Montbell UL Down Parka (total 9 oz for a Medium, with 2.5 oz of 800 fill and a hood) should be comfortable enough for 28F to 30F.

At the lower range (18F to 20F) you would be pushing their limits, I think. Much would depend on your other clothing, such as gloves, or a balaclava, and insulation on your legs and feet. Plus, your activity level. If the temps will not stay that low for very long, as the day warms up, then the risks are definitely lower, even if you get a bit chilled.

I would hope your sleeping system is capable of keeping you reasonably warm at 18F, because you say those temps are possible. During sleep you generate much less heat than when awake and moving about.

Edit: As you may notice, people differ greatly in how easily they get cold or hot, so as always, it is best to know your own body and how it operates. I tend to become cold rather easily, but heat up to 90F does not bother me.