RE: Digital throttle on throttle?
I have been using Hitec digitals on throttle for many years now never a problem all the way to 40% models. I don't suggest the metal geared versions. The Karbonites are my preference or nylon geared precision coreless servos.
Throttle requires precision repeatable results, in precision and 3D flight the throttle servo likely gets as much a work out if not more than other surfaces IMO.