I can tell you from experience that a 91 will not hover the GS300, which is very similar to the Extra. I have a Kange Staudacher GS300 and have a 91AC in her. She flies but on the wing and will not come close to hovering. She weighs in @ 9.5 lbs. Wet. I am in the process of installing a YS 120NC now so I can tell you more next week.