I'd forgo any weight in the stock, would imbalance the gun. I don't even like a butt cuff with ammo on my guns when hunting. Balance and feel are important to me.
Most persons I know refer to 3" as "magnum". I've never seen 3" low brass dove loads, after all. I have a box of federal 3" in front of me right now that says "Classic STEEL Magnum" on the side of the box. If it's a 3", it's a magnum. True enough, we used to call high brass duck loads in 2 3/4" as "magnum" and "magnum" was often printed on the box as well. But, I have a box of Winchester Xpert Hi Velocity steel 2 3/4" in front of me now, says 1 1/16 ounce 1550 fps. For all intents, that's an old time 2 3/4" magnum load, but I do not find the word "magnum" anywhere on the box. On another box of Xpert, 1300 fps 2 3/4" 1 1/8 ounce, it actually says on the side, "waterfowl and heavy upland game loads". Remember, this is steel shot and 1 1/8 ounce is about all the steel shot a 2 3/4" hull will handle. You simply don't find the word magnum used anymore for 2 3/4" loads, it seems. You do on 3" and 3.5" loads. Like I say, I've never seen 3" dove loads.
The word "magnum" is tossed around a lot. Some loads that probably should be called "magnum" are simply called ".454 Casull" or ".480 Ruger", yet some that are laughable as a hot load are called ".32 H&R Magnum" or .22 WRMagnum. I don't think there's a real hard and fast rule for the use of the word. It is what you want it to mean.