If you get wet, you're probably gonna get cold.
I was worried about the same thing. My first UQ was a synthetic one; however, it didn't pack down very well. I talked to some vendors & they commented that if you're under a tarp, your UQ will probably not get wet. Well that wasn't entirely true, because I've slept in clouds blowing in off the Puget Sound at elevation. But, I have several full length down UQ's now & I wouldn't go back to a synthetic or a partial length.
The down holds up well, as long as you keep them unpacked (like any good down sleeping bag) when not in use. I love my HG Incubator 0*s. I got canvas sacks on Amazon to store them in when not in use. (They're on sale till tomorrow too).
I did get a few of 2QZQs UQ Protectors & recommend them. Even if you don't have blowing rain, they still cut down on the wind blowing the heat away from your UQ. I think they'll add about 5* to your temperature rating.