Hi,
basically, you get better performance if you do not have to limit the servos too much by way of reducing collective and cyclic, in setup.
18 mm sounds much, 16-17 would be better. Would it be possible to drill new holes further in, cut threads, mount there?
On the other hand, current digital servos can cope pretty good, so it's always worth a try. It's not that the model would fall out of the sky, it's just that control loop performance is better if the control loop and servos can breathe freely, figuratively speaking.
Kind regards
Eddi
Born to fly ...
forced to work.