Pictured is the only group I shot off a rest, I'd guess ~1.7 inches. I shot several other groups before getting a bag out to see what loads had potential. In an odd position sitting with my back to a wall the other groups were barely larger with 150gr syntech, 115gr gold dots, and some Underwood penetrators. So far the fit barrel seems to just about cut groups in half in terms of maximum potential. I had a 43 out today for the first time and it shot the lighter 115gr syntech better than the full fit barrel 19 shot 115gr syntech (awful ~7"). The 43 did a ~3.5" group with 115gr gold dots which is about as good as my factory Glocks ever do. Not particularly a 115gr gold dot guy, it is just what I had for accuracy testing.
I am delighted with the results and will test more loads soon. I'm not sure how much of the ~1.7 inch spread is my human error.
I tried KKM, Silencerco, Faxon, Backup Tactical, and Griffin drop in barrels. I would go KKM as far as drop in, but now I think A fit barrel is worth the trouble.