I had a KKM (drop-in) in my G34. The only reason I had it was to be able to shoot lead.
The only improvement over the stock barrel (to me) was the conventional rifling, more chamber support and a slightly tighter chamber.
As for as accuracy, to be honest the factory barrel was slightly more accurate than my KKM. I'm sure a fitted barrel would be different.
On a side note: I just purchased a LWD barrel for my G21 and so far it has every advantage as the KKM but at a cheaper price.