What can it do, that the SP can't? On the regular samples do not stop when you let go of the keyboard. You can neither sequence other gear which requires note off, like synthesizers. The SP MIDI address these issues.
Can you use multimode to control a synthesizer? You can trigger notes with multi pitch and D1-D6. The SP will no longer mute notes right after they are triggered.
Can you use multimode to pitch the S900/S950? Yes, you can, on multiple samples. You can also use the loop functions.
What about channel voices/polyphony on the SP? Internal samples are still monophonic and use their assigned voice channel. The external sequence (bank D) use one channel total.
Is it possible to use decay/tune simultaneously? No, not really, but a fast tap on a keyboard will give a short hit/instant decay.
Can you fine pitch? No, unfortunately not.
The SP MIDI features firmware updates via SysEx messaging. The user can update the firmware using a computer equipped with a standard MIDI interface. Report a bug!