It isn't so much that the 20 gauge is great, as much as steel shot is terrible. Does steel shot kill within its limits? Yes. Is it even close to any other kind of shot? No, not at all. If all a person did was decoy birds and shoot them inside 30 yards tops, then steel is fine. Beyond that and it just isn't it. I also don't believe steel shot can ever really be effective on Canadian geese, not at 40 yards and beyond anyway. "I shoot BBB, it breaks their wing" That's all I heard growing up. That's about what you can hope for too.
Sorry, rant over on that. Yes, a 20 gauge can work fine. A lot of the tungsten loads are around 12 g/cc which is very close to lead shot, a smidge denser actually. That's what makes the 20 gauge really viable for ducks and geese beyond spitting distance. I gave up on steel shot about 2015, and have gone almost exclusively to bismuth shot which I've been loading myself. Even today there are not many bismuth loads out there that are honestly good. No, I have not seen any impressive patterning results from Boss shells either. Bismuth behaves a lot like lead, and as such it really performs best at moderate or low velocities. You just are not going to get peak patterns from it at 1400 fps. Buffer is also a really helpful thing, which nobody except Winchester is using right now. That said, I'm not sure bismuth is as impressive in 20 gauge as you would be hoping for. I have a really mean buffered 20 gauge load of 1 1/8oz at about 1275 fps. Best shot size would likely be #2 for Canadian geese in this load. It would probably work at 40 yards, but not much more than that. It's much better as a duck load with #4, still probably not much beyond a 45 yard load. #5 bismuth will not get the penetration you need at that distance. I've had really poor results with #6 bismuth on ducks. It would probably be great on teal and similar, but on mallards you are not going to reach the vitals unless they are really close. I don't consider head shots anything other than luck. If a pellet can't penetrate to lungs it fails. I can't disagree more on the choke. If you use a more open choke with bismuth, and patterns tighten, your load is really, REALLY poor. You are almost certainly shooting one of those 1400+ fps wiz bang nonsense loads. Treat it like lead, get a proper load under 1300 fps, nothing wring with 1150 fps either. Choose a buffered load if you can. Bismuth responds really well to choke. Unless I know I'm getting close shots, I use full choke. The only shotgun I have anymore with removable choke tubes is my Benelli Nova, and it performs best with a turkey choke. All of my loads pattern about 95% at 40 yards with a full choke.
My advice is if you want that kind of performance from a 20 gauge, stick with tungsten type shot.
). 
