The best Upland hunters I've shared with have favored a wide assortment of shotguns.One of the best was my old buddy Billy, whose Polychoked A-5 was nose heavy as a pig on a snow shovel. Another favored an AL 48 in 20 gauge. I've carried lunches that weighed more.
A couple shotguns I've owned made me look like I knew what I was doing. These included riot barreled 870s, a 20 gauge SKB O/U, and a little French/ Belgian SxS that weighed 6 lbs, 5 oz, was a straight gripped, beautiful death ray with a stock sculpted by a master. Wish I still had it and the SKB.
What good upland shotguns have in common is they handle like Zorro's rapier, while waterfowlers and trap guns handle like Braveheart's two handed can opener.
It's not so much the weight as how the weight lies along the axes. When more than 50% of the gun's weight lies between the hands, it makes the thing more responsive and easy to get into motion. A slim design like most SxS shotguns keeps more weight close to the COG and aids getting that mysterious sweet "Feel". The kind of shotgun that when one takes that first shot,we look down at it and smile.
Downside, of course, it that these are easier to stop also. No inertia to keep the swing going.
I've little grouse hunting here, I believe my lifetime total is 3. But if I were going to get a purpose made grouse/woodcock/quail shotgun, it'd be something like....
Capable of handling at least 3/4 oz of shot.
Choked for a good pattern at 20 yards in the first barrel. And for 30 yards with the second. Or 25 yards with a single barrelled gun.
Less than 6 1/2 lbs.
And pretty.....