It's all about the terminal energy transferred to the target, in this case, the head/neck of the turkey. If you can put a swarm of #6 shot on the target, that will deliver all of the energy, killing power, that you need. Having used 3" WLB shells with 1.75 oz of #6 shot in competition for the last few years, I can testify to the ability of that shell to do just that. I have seen some absolutely amazing patterns, numbers that we would not have even thought of just a few years ago. Chokes in the 6.50 - 6.55 exit diameter have been best.
Thanks,
Clark