I started with a 20 gauge 25 years ago and hunted with it exclusively taking 100+ birds with it using lead 5's. I went to various 12 gauges after that and they all killed birds too. The last couple of years, my 20 gauge has sat in the safe while I carried a 3.5" Maxus with Mag blends and it absolutely hammered birds.
One thing I noticed though was me not getting the Maxus up into position as early and not holding it up as long.
I bought a 28 gauge Bobwhite setup with TSS 9.5's to go to next season. It weighs 2.5 pounds less than the Maxus empty.
All that said, I have only felt under gunned a couple times and all of those I had a 12 gauge in my hands with lead shot.