That autopilot functionality has been about for a little while - even I have it
(down to Cat 1 i.e. 200ft, officially)
Auto throttle is a complicated bit of kit and I would be suprised if somebody did it in a piston GA plane soon. One reason, I guess, is that when one has an autopilot which is tracking a programmed VS or an ILS, one has a low cockpit workload anyway and it is trivial to adjust the airspeed using simply the throttle.