I'm not sure if that sounds familiar to me. In terms of vital anatomy, let's use this as our reference. For scale, the human heart is roughly the length/width of your open hand.
As you can see, basically everything in the chest is highly vascular. If you throw a projectile into the top of the chest - especially a 1oz slug - you are going to put someone in a world of hurt.
While it is conceivable that a wider spread might put a 00 pellet into the heart where a slug or tight shot pattern might pass right by, but remember:
1) everything in that area is extremely fragile (pulm A/V/hilar structures, aortic root, thoracic aorta)
2) as pellets spread and impact individually, they quickly lose momentum because of their rounded shape. A round, slow-moving, soft (lead) projectile is far less likely to penetrate a hard, bony structure like the ribs than something else with greater momentum.
I have never seen anyone who has been shot in the chest not go to the OR - even with just a handgun. Basically, I really don't think having a "wide enough" spread is something to worry about. If all 9 pellets of 00 buck connect with the head or torso, your target is not likely long for this world, no matter how tight the spread is or not.
To qualify, I am still very junior in my training, but I've seen probably somewhere around a few dozen GSWs. Always open to correction from others who have a bigger piece of the puzzle.
No worries, and I hope my reply wasn't too rude or dismissive. There are lots of ideas out there and I think most all of them are worth considering. Only adding my piece of the puzzle