SX600 comes with new styles, a few taken from the CVP 809 and also several new features like the Unison & Accent feature. Doesn't make sense for a lower SX600 of the same SX series to have more exclusive features and content than the more expensive & flagship SX900. So I think we can expect a feature update on the SX700/900.
We saw this trend with the Chord Looper function where it was a launch exclusive feature with several exclusive styles on the SX900 and 6 months later, Genos got it.
But again, take it with a grain of salt.