I didn't see velocities listed for Polywad, so couldn't calculate.

But if you run the numbers, you'll see a standard target load of 7/8 @ 1200 generates about 15 ft-lbs in your gun. Remember what that felt like?

Then RST's 20ga 3/4 @ 1150 generates about 10 ft-lbs, or 2/3 the standard load.

And RST's 28ga 5/8 @ 1175 genrates 8 ft-lbs.

4-5 pellets from any of those (or a .410) will kill the grouse....how good a shot are you?

If I couldn't increase the gun weight, I'd be tempted to go with the RST 20 ga load. It could be bested a bit with a handload of 3/4 @ 1100 fps (only 7 ft-lbs), easily doable.