I am trying a Sublite(Tyvek) this year for the first time. I haven't encountered any precip so far, so can't comment directly on its storm worthiness, but Henry assures me that it is perfectly adequate for typical Sierra rain storms. Still, the proof is in the pudding. I am heading into McGee Lakes in a week or so and that area typically encounters more precip than my usual Southern Sierra haunts. I will report back to you on the results one way or the other. If it proves up to the task, it would be a very attractive option at 20 oz, because it provides bug and rain protection with minimal/n condensation. The thing I don't like about a bivy, even with a built in bug net, is that it is very confining and can have a lot of condensation buildup. OK if you are thru hiking, but if you happen to want to hang around a buggy area for a day or two to fish or day hike, it is nice to have a place to retreat during the day that isn't like a coffin.
My 2 cents.