Kevlar in this application does not in fact stretch to make any difference in control. But it does relax over time and will probably need some minor retightening. It will work fine. I've used Kevlar (braided material that I built and covered myself with teflon (not for sale so don't even botherto ask)) for at least 25 years.

In a pull-pull rudder application, any Ackerman effects have been negligible......the pulling side always is tighter than the other side. Tension is very light, just enough to be snug, but not tight. No guitar strings needed.

I use pull pull on elevators too because it is the lightest arrangement and allows 4 points of adjustability.