Middle position
I too ran into the same problem on my -7, so I called Garmin. They said that the middle hole was the ideal position, as it meant that more force could be applied by the servo on the pushrod than if using the outer hole. The downside is that you have to be more accurate at adjusting the pushrod length, otherwise it will but up against the servo stops at the extent of the aileron travel. Get your ailerons adjusted first to match your desired throw. Keep in mind that the neutral position of the stick is about 19 degrees of forward elevator (IIRC), as this seems to affect the aileron position through slight geometry changes to the large aileron pushrods. Then adjust the servo pushrod length so that at each of the aileron stops, the servo arm has equal clearance to the respective servo stops. This will mean that in the neutral stick position the servo arm will NOT be perpendicular to the servo body, just by a few degrees. The important thing is that at the extremes of the aileron throw, the clearance to each stop is the same. If the neutral stick position results in a perpendicular servo arm, the arm will contact one of the stops at the extent of travel when using the middle hole. This is why many people just use the outer hole. The total linear travel is the same, but the angular travel using the outer hole is less, meaning that the alignment with the servo stops is less critical, but at the cost of the servo being able to provide less force to the control surfaces.
On the topic of the servo stops, the instructions say to let half the stop ride up on the main servo mounting bracket. As an engineer, I can not stomach this half baked solution, so I have trimmed the bottom of the stop bracket flush with the servo mounting bracket, creating a left and a right stop. These now mount separately, but flush, with the face of the servo. I have performed load calculations for the mounting screws with the bracket in this separated fashion and they are more than adequate to handle the maximum torque of the servo. Your opinion (and obviously Garmin's) may differ on this point, so I would advise that you perform your own basic load calcs before proceeding accordingly.
I hope this helps.
Tom.