I have each of the Glock pistols in 10mm- G29 (2), G20, and G40. I had the G20 for a while, and loved it and got the G29 to carry. The G29 is my main carry gun. I also put on the 6.5" Lone Wolf barrel on the G20. It's ok, and I'm not seeing a huge gain in difference from the factory barrel. The one thing I do notice is the Lone Wolf has a tighter chamber, and will up your speeds a noticeable amount. Yes, you will gain some speed from the added barrel length, but the gain comes more from the tighter chamber. I have also noticed I do get a few rounds that will hang up and fail to chamber when shooting with the Lone Wolf. They will porpoise out of the mag and hang up at an angle. I'd like to try the KKM barrel. If you go to a custom barrel, I'd recommend the heavier spring. Mine is factory spring, and a standard max 10mm load is a heavy recoil in the custom barrel, and may be too much for the factory spring.
But, I would also recommend the G40. It will out shoot my G20 with the custom barrel. The added weight of the G40 helps tame the recoil a lot. The G40 is now my favorite pistol to shoot. We were punching steel at 100 yards with it a few weeks ago. When we missed, it was my fault on the trigger. It is a VERY good pistol!