Curious as to why you would use larger shot for turkeys? I-only target the head, and even 1 small shot in the brain should do the job. I have killed more than 20 turkeys with a muzzleloading shotgun with #6 shot, out to 40 yards with one of the guns. With a 20 gauge smooth rifle, I used #7 1/2 shot with good results at 20 yards.
1 ounce of # 5 shot, has 170 pellets. The same weight of #2 shot only has 87 pellets. That would reduce your chances of a fatal hit by almost 50%. A load of # 7 1/2 shot will easily penetrate a turkeys head at 20-30 yards and contains 350 pellets. Twice as many as the #5 load.
Not trying to start an argument, but I have always tried to maximize the number of shot on the small target that a turkey head offers.