Actually the .357Sig barrel is NOT even a conversion. Just buy the standard barrel for the .357Sig cal gun and it fits your G23 or G27 and you even use your same .40 cal mags, and it's also backwards compatible, that G32 and G33 Baby Glock .357Sig will take the G23 and G27 barrels and use your same .357Sig mags for your .40 ammo. This is why when buddies or family ask me which Glock to get, I tell them G23 First Choice, if they don't have it in stock, Then see if they G32 .357Sig in stock. Or the G27 and if it's not in stock, get the G33.
I really see no reason in getting the Full Frame G22 or G31, unless you really want the big gun. You're giving up conealability by going full frame, and with mag spacers like Xgrip, you can still shoot the full frame mags and extend your grip that way too for range shooting. I have a G23 with 2 G23 13round Mags, and 6 G22 15 round mags. Xgrip spacers are on the way.
This is what my G23 looks like with G22 15round mags in it using the spacers.
Now there is a 9mm Conversion barrel for the .40/357Sig Glocks. That Conversion barrel is also backwards compatible and fits BOTH the G23/G32, G22/G31 and G27/G31. So with a G23/32 and $250 for 2 more barrels, you can shoot 2 more calibers. However, you do have to use the 9mm G17 or G19 mags. This is also why I have 3 G17 17 round mags.