Yeah I have def thought about that. 40 ammo is a little pricey.
I would recommend spending a cool $100 or so on a lone wolf 9mm conversion barrel. Cheaper ammo to play with and can drop in and go. I even use the same .40 mags and no feed issues. I love my g23!
Be careful if you are thinking about carrying it with the conversion to 9mm barrel. Since the 9 case is slightly smaller than the .40 there is a slight chance that the extractor could fail to properly pull a round out of the chamber. In training, no problem. In self defense carry, oh oh!I had the conversion on mine. I actually shot better with it than the factory barrel. Hence why I switched to a G19.
I recommend those conversion barrels. They are great. (Dont forget to buy G19 mags to go with it. G23 mags wont feed 9mm properly)
My all time favorite holsters are the Horsehide OWB Scabbard from Kramer Gun Leather. They are pricey and take months to deliver, yet are simply the best I've found so far. When carried with a good gun belt, it keeps the gun nice and high and tucked tightly in to your side. Wear that under an untucked shirt, and it conceals very well. I carried my G23 to the state fair twice this year with that rig and had no issues at all.
This is the holster I use: Leather Gun Scabbard | Kramer Handgun Leather