So, aren't you back to square one again? Using the outside hole on the horn makes sense, or making sure your horn is setup for the neutral of the servo also makes sense to me to lengthen your throw.
However, doesn't it then require you to recenter the surface either by trimming or by manually trimming (resetting the linkage on the servo)?
I am interested in increasing my rudder throw on my World Supersport .40. If I move the clevis closer to the control surface it lengthens the control link.
If your control horns are properly placed on the control surface, where the holes in the control horn line up with the control surface's hinge line, you should not have to adjust the linkage. If they are not lined up with the hinge line, then you probably will have to make an adjustment. When installing control horns, you should try to make the holes in the horn line up with the hinge line of the control surface.
Mike