STEC autopilots were never much good for performance because they use the TC as the roll reference, and use indirect means as the pitch reference.
You're a bit unfair here Peter. In fact, rate of turn based autopilots do work quite well for smaller aircraft. In most situations, there will be no real difference. In slow flight, they tend to oscillate because they have no means of determining bank, only the resulting roll condition. Above 60 KIAS, they usually do a very good job. The pitch axis is controlled using ambient air pressure (and its delta over time for climb/descent) as well as accelerometers to detect turbulence. I agree that attitude based autopilots are superior but they are also a lot more complex and -- the biggest drawback -- they have a strong dependency on a certain AI/HSI. HSIs are very expensive to repair and you can't easily replace them with something else when you have a KFC225.
The big difference between S-TEC and KFC225 is that the former is a retrofit autopilot and the latter only comes (or rather came) with factory new aircraft. S-TECs are field installed (with varying quality) and you find them in a lot of aircraft. If installed well, they do their job but often they're not installed well. In my airplane, I cannot complain about its accuracy and comfort level -- when it works! The Cirrus aircraft were equipped with S-TEC until some years ago and they work well. In your case, the aircraft manufacturer installed the KFC225 and flight tested it. That is of course a much better package than a retrofit autopilot that was mostly paper tested.