I actually have a G21, full size frame, but....
Have considered getting the 10mm barrel, but I've also researched this a bit more and as far as I can tell the G20 was specifically built around the 10mm cartridge, and the G21 was not/is not.
I'm pretty sure the Glock 21 is just a Glock 20 in 45 ACP. I remember reading an article talking about the Glock 21 being good for a 45 Super conversion because of this. I'm not a big Glock guy though so I could be misinformed.