What can a gunsmith do to a stock Glock Model23 in .40 cal to make it a better more efficient weapon ?

Not much more then you can do your self night sights some polishing on the trigger bar if necessary.

I own 3 use them out of the box as they came with night sights.

Buy ammo and practice, practice, practice.

Drop-in barrels might tweak the accuracy potential.

Some people change the plastic sights to steel sights or night sights. I did this on my Glock 17, but I like the stock plastic sights with the white outline in the back myself over 3 dots.

I suppose the next time I'm in a gunfight and shot in my primary hand and trying to rack the slide one-handed against a desk or wall corner, and can't catch the edge of the slide, but slip to the edge of the front sight, and the plastic sight comes off and I can't chamber a round, I may come to the realization I should have swapped to a steel sight, but...

I have a KKM barrel in .357 Sig that I swap into my Glock 23. Use the same .40 S&W magazines. Incredibly accurate, and no hang ups with probably 500 .357 Sig rounds through it. That would be my upgrade recommendation.

off topic.

I have a 357Sig and 40S&W barrel for my G20. Looks like Lonewolf doesn't make them anymore.

Many of the factory issue Glocks shoot extremely accurate. I have a first gen 45acp that will stay right with many gunsmith built 1911's. I was shocked when I first shot it. Still have it,it's keeper.

There are trigger assembly parts and springs that tune the whole system and a much better way then polishing the existing setup.

The plastic sights aren't that bad and not easily damaged as many claim.

LoneWolf quit a long of things. I bought a Glock longslide from them in 10mm. I don't believe they catalog that anymore either. Shoots great.

My recent Glock was to gunsmith a Model 17 with the aforementioned tricks. I put a reflex on it and it's pretty dang accurate for a 9mm. The trigger is some much better then the stock ones or even the polished stock ones.

was looking for trigger work seems like a lot of slack in the trigger

I would start with a 3.5 connector and a trigger job. You won't get much of the trigger travel out.