Hi Arnaud,
I bought mine with the STC already implemented, but my understanding is that a change to the 3-bladed Hartzell prop is a normal part of the upgrade (especially if you currently have the 2-bladed McCauley).
Almost all Ovations run LOP perfectly fine without GAMI injectors. My GAMI spread is close to 1gph, not enough to bother with the extra expense of getting GAMIs.
Robert