There are two main questions here:
1) Is a WPB shell a good idea for a sleeping bag?
2) Is WPB cuben a good choice for a WPB shell?
A WPB sleeping bag shell is better at avoiding external moisture (ie. rain, melting snow) but worse at dealing with internal moisture (vs. a non WPB bag) since breathability is reduced. I believe the general wisdom here is that WPB is not a good idea, but there are different ways to approach this. With care and skill, one can reduce the risks from external moisture with a non-WPB bag, but internally generated moisture will always a big concern with winter use - an issue which WPB bags amplify. So a WPB shell eliminates a controllable problem but amplifies a difficult to control problem.
Body generated moisture often freezes within the bag as it gets further from your body, leading to an accumulation of moisture over time and a loss in loft if not remedied. This is often why people use vapor barriers, which you could employ within a WPB bag. Most people either go on trips that are short enough that accumulation isn't a big deal (ie. 1-2 nights) or for longer trips they take advantage of warmer periods (ie. sunny afternoons, campfires, hot tents) to dry the bag. With a WPB bag, it's going to be tougher to dry since the breathability is lower, so you'd be at a disadvantage here.
Often when I winter camp it's not too cold when I first head to bed, maybe 20 F. In these temps it might be 30F in my tent, which is a situation where my body can heat the bag enough such that vapor can exit before it freezes in the bag. Thus if the bag has good breathability, I can reduce some of the moisture that was accumulated the night before. With a WPB shell, it's less effective to take advantage of less cold periods to reduce the moisture content in the bag. Once it's in there, it's very difficult to remove on the trail. Accordingly, I prefer to take care not to get external moisture on my bag (no WPB shell), which allows me to take a lighter and better breathing bag.
As briefly mentioned, WPB shells are heavier. I think WPB cuben is around 1.4oz/yd, while the lightest shell fabrics (ie. 7D nylon) are about half of that.
Regarding the second question (ie. WPB cuben a good WPB choice?), I imagine it is. I've never had difficulty with even the lightest fabrics for sleeping bags, so I doubt that durability would be a concern. Breathability is another issue, and think the WPB cuben is generally regarded well in this respect. It's is pretty darn expensive though, and its also not readily apparent why its better than something like Pertex Shield+. I imagine Pertex Shield+ is lighter, less expensive, just as breathable but potentially less durable which I don't consider to be an issue for sleeping bags.
A lot of people prefer to use a second layer/blanket over their sleeping bag such that the moisture can exit their bag and collect in this less crucial - and more easily dried - layer. If you do longer trips in the winter, it might be better to go with a non-WPB bag (higher breathability) and add a thin synthetic blanket on top with the money/weight you save vs. a WPB shell. This setup wouldn't be as effective against external moisture obviously, but it would be far superior for internal moisture which is generally the more relevant concern for longer winter trips where the hiker has the maturity/caution/skill to avoid dunking their bag in a snowbank.