You won't save a massive amount changing from beech to walnut (about 1 to 2 pounds) the Prosport is just one of those guns you'll have to get used to the weight, yes it's a beast to carry around all day but the balance and performance outweigh the weight problem, I have a very early Prosport in a later walnut stock and it's fantastic.
They also look far better in walnut and if you do happen to knock the stock you can easily rectify the problem more easily with walnut.
Pete