I went a different direction buying a fully-supported chambered barrel for the Glock 21 and got a 24# recoil spring and am running .45 Super from Buffalo Bore and .450 SMC (the small primer .45 Super ammo from Double Tap) and am getting some pretty good ballistics at 1450 fps and 800#+ of disbursable energy!! Plus, I can still shoot .45 acp through it!I did that with my 41, KKM 10mm barrel. Now I shoot 40, 10 and 45 through my 41.
You could always buy a 21 and get the conversion parts and have two awesome loadouts for your gun. Just say'in.