I would first try spreaders. They will likely surprise you and open things plenty for grouse/wc.

If you plan alot of shooting the cost of factory spreaders may be a concern. You can purchase spreader discs/inserts and load your own. Its really pretty simple.

If you open the chokes I pretty much agree with the others. I would go .005 and .015 for a dedicated 12 ga grouse/wc gun.

Sam