I have 20 set up for my kids, I think if you look at the ballistics, some of the hotter mag sabots are pretty much equal to a 454 casull, I don't think you would hear anyone complain about shooting a deer with that round, at least not from the perspective of not having enuff power.
The only problem I have w/ shooting deer w/ a 20 is when using buckshot. Just doesn't have big enuff pellets or enuff of them. Both my boys killed their first deer this year. Both w/ 3 inch buckshot. The first one dropped where he stood, the second I found the next morning (had one magic bb through the top of both lungs). I am just not confident in the buckshot loads. I have since put an aimpoint and sighted it in w/ slugs. Very accurate to 100 yards and will be what we use next year for the boys.
R
Hank