This is the type of move AA has made many times in Atlanta. He signs guys young, and most of the time it works out for him.
The time to get best value on Davis would definitely be this offseason, if he carries this run to the end of the season.
If the K rate is around 25% and the BB rate stays above 15%, and the power continues, I would definitely try and extend him. I don't care if the BABIP luck starts to decline and his doubles and singles start finding gloves, if the process and approach is the same, and he's still hitting dingers, you absolutely should try and extend him imo.