As of now, HSM / Ultrasonic motor on Pentax is pretty much un neccessary. AF is a total system affair. Not only just the focusing motor. There is the drive mechanism, the AF governing circuit and drive circuit as well as Focusing software/firmware.
Truth is Pentax is kind of lacking behind on this matter and need some real improvemetn to be able to start making any inroad. Pity though as all other major Mfr now have this pretty much work out !!