As in all of life, depends on your usage.
If just the outer shell is waterproof, then moisture can accumulate in the down degrading it's performance anyway. Over a few days in freezing temperatures this becomes a severe problem as the down loses loft and the moisture trapped by the WPB membrane freezes, increasing the weight while decreasing warmth. If the entire bag is water proof (like the cuben bags from some cottage manufacturers) this is less of an issue, although the VBL effect may be unwanted.
As for bivies, 550g is a lot. MontBell makes a 6 oz WPB bivy that uses their breeze tec membrane that is often compared to just a touch less breathable than eVent. May be worth looking at.