I have a G20, a few conversion barrels, and I've been following the threads on this subject for a long time.
The most reliable accounts I hear about it have been from all the competition shooters who rave about KKM barrels. That's a brand I don't own, but the fact that so many guys win contests with them says something. I own 2 Bar Sto's, and consider them to probably be second best to KKM, however Bar Sto is expensive, and you could even say over-priced. I also own a couple Lone Wolf barrels, but compared to Bar Sto & KKM the rifling in a LW looks rough and sloppy. I also never heard anyone else say they won a contest with a LW barrel, or a Storm Lake either. Some people on GT have said their Storm Lake barrel was well made, but I also hear people say it was the worst looking barrel (inside) they ever saw. Lone Wolf barrels seem to attract people because the price - less than half what a Bar Sto costs - and they are certainly the cheapest barrel you can get if price is a big concern.
As far as actual tested accuracy, I'm afraid I cannot give you what you want to hear because I'm a geezer with bad eyes and can't see my sights well enough to do a truly valid accuracy comparison. Some guys have said on GT that even a lone wolf barrel improved their accuracy, but many others have said it made no difference. KKM is the one brand that NOBODY ever says anything bad about, and the only brand with so many serious contest shooters claiming to have won matches with them.
What is wrong with the OEM bbl? It is as good as the shooter (probably better). Is the Glock 20 supposed to be a target pistol? If you want a target pistol wouldn't a tuned .45 ACP make a better platform?