You'll be fine with those servos. I started flying pattern planes this size with the JR DS821, which is basically the same amount of torque. It is a good, precise servo with plenty of power for the ailerons. Probably a little slow for me now, but I would not hesitate to use one for a backup in this plane.

I did a lot of similar modifications to my plane - titanium turnbuckles with ball links, lots of other things. What I found is that it was really a waste of time, money, and brain power. The plane flies great right out of the box, that's half the beauty of a Sebart plane.

This is a hobby, so if that's what you really enjoy, then go for it. But in hind sight, I would have had more practice, less problems, and more money in my pocket if I just flew the plane the way it was meant to be flown. That being said, if you really want to change hardware, you can find ball links with smaller screws, and maybe only have to shave the control horns.

I did the exercise of drilling a lower hole in the composite horn of a different plane, for the same reason. It works, but I've never been happy with the results. It looks like crap, it's difficult to get both sides to be exact (thus affecting geometry similarity), and changes the angles of the setup. I believe it is better to use the existing holes, enlarging them slightly if need be, or using a different control horn all together. Just my own thoughts from my own experience