I'd choose 10 mm myself as it's a definite step up from 9 mm or 40 S&W.
Checking commercial ammo, Hornady's 180 gr XTP 10 mm has an advertised MV of 1275. So there's at least one full power load by SAAMI specs.
Note: I'm a G29 owner and I usually load 180 gr to around 1100 fps. I'd say a G20 with the above load will get your attention and the attention of the shooters next to you.