The architecture of the SP-16 is pretty set. There are fixed amounts of ram for the pads so adding more pad banks seems unlikely. And because some of the pads can already be MIDI, adding a new MIDI bank causes problems as there can only be 16 midi tracks for the single midi out.
Honestly, don't judge the SP-16 on the specs. You have to use it, get its workflow and judge it on its merits. In real life it is far more intuitive than reading the manual suggests, the step modulation is far more powerful and instant fun than you might guess. They took some simple ideas and implemented them really well. Try to use one before you dismiss it.