I bought a KKM barrel for my Custom Glock 20 and KKM will tell you if you shoot Underwood, Double Tap, Buffalo Bore, then they need to make a slight modification to the barrel which they do for free. I had this done and no issues at all.
I will be buying a KKM barrel for my G40 probably next month.
I haven't used Lone Wolf barrels but for $165 the KKM is very accurate and groups the Underwood 220gr hard cast lead into
nice tight groups. No smiles on the brass so that's a plus.

I have both the G20 which is heavily customized and the G40 which is currently bone stock.
I prefer the G20 over the G40. I dislike the G40's MOS system. I thought I would like it but
it rides the MRD too high to co-witness and I'd rather go with an Aimpoint T1 but can't because the
MOS system doesn't allow it.
Might buy a new slide from Lone Wolf if they can mill a picatinny rail onto it.
Lots of bullseye shooters use tube red dots and after using a Trijicon RMR on my G20,
I think I'd prefer an Aimpoint on the back.
Also if you want to shoot lead, the stock G40 barrel has to go, mine keyholes with lead. That's
not unexpected but it's another $165 to KKM.
Longer sight radius is good if your eyes are good, mine are not any longer...

I have a Glock 40 and it hates lead ammo and tends to keyhole with it. So I will probably get a KKM barrel for it.
The gun itself is kind of nose heavy and doesn't feel as balanced as the G20 which I also have.
I really don't care for the MOS system and have been thinking of changing out the slide for a lone wolf long slide.
I think I would rather have an Aimpoint T1 on the G40 rather than a Trijicon. I don't care about co-witness on
a hunting/woods defense gun.