Well, I checked this out on my E433, and from what I found, when you change the split point, it affects all of the music database styles at once, and it is even stored when the power is turned off and back on. Since the music database is just a collection of preprogrammed styles and voicings, the split point affects all of the music database styles just like it would affect the keyboard if you manually chose a specific style and set up your own main and dual voices.
I do believe that the split point is saved in a registration, however, so that different registrations can have different split points.
The split point affects both the point on the keyboard where the auto-accompaniment chords take effect, as well as where the actual split voice is played.