I don't see the light wheels being an issue on decent. They still have to spin up and heavier wheels are going to spin up slower even going downhill.
The extra little bit of weight of a heavier wheel is not going to help you much going down hill. It adds a little more potential energy but this is lost to the increased rotational energy being sucked up by the wheel.
On the climb, the reduced weight helps a tiny bit in the (less weight in general is better when climbing) and the reduced rotational energy is a positive too.
Bottom line, don't worry about the affect on the downhill. I don't think you will be able to see any difference.