It sure is amusing to see all the different opinions regarding chokes and loads for grouse. For what it is worth, here is what I subscribe to in my .20 side by side:
Early in the season I use extremely open chokes...usually Skeet/IC
After the leaves fall, I go with IC/Mod.
For loads, I always use 3" copper plated #7.5's. For some reason, I am partial to Fiocchi loads (I use their #5's in the pheasant fields with Mod/Full).
One thing I should note is that I rarely get a shot at a grouse going straight away. Most of my shots are crossing or at an angle and usually not all that close...25-30 yards or so. The reason is I hunt with a Labrador Retriever and it is pretty much a run and gun proposition when she gets birdy!
I saw someone mention that often a grouse will flush after you have stopped walking for a few seconds. This is also true for the second grouse....after you shoot at one, always reload before you take a step to look for signs of a hit. That 2nd bird is ready to jump. Good Luck!