The key is learning how to shoot and shooting a gun that fits. I shoot as many geese now with a 20ga as I did years ago with any of the 12 and 10 gauges I owned and shot. Learning range, lead, follow through and shooting good combinations of choke and load will fold birds. Invest in lots of off season practice on a skeet or sporting clays course and she'll find her 20 is plenty for geese. My wife shoots a Beretta A400 Xplor 20 gauge. I got her into trap and then skeet to teach her how to lead, follow through to center birds in the pattern, learn leads at various angles and took her to a NSCA Level 2 Instructor to have her gun measured and adjusted to fit her and some professional instruction. It was well worth it. If you can't centre a bird in the pattern it won't matter a hill of beans the size of gun and payload.