the piano in the KN7000 is the identical technology but there are just detail differences in the implementation.
the p50 is bought by people wanting a stage piano with 88 keys weighted like a real piano, heavy at the bottom end and lighter and undamped at the top. The wave rom is used for a handful of high quality sounds, so without any backings built in, the 64 polyphony should never reveal any drop out.
the KN7000 has standard synthesiser 61 key plastic keys with aftertouch, with a rock solid 128 polyphony. The grand piano multi-sample is particularly good, but this machine is designed to do everything from style generation, multitrack sequencing, sound editing to digital audio generation, a complete digital workstation.
many professionls buy both, use the KN for backings and the p50 for its voices and as a midi controller for the KN.