SSE2 is needed because of speed issues. PSX is a much simpler piece of hardware so it doesn't need a modern computer with SSE2 in it. Every current gen computer has SSE2 by now so I see no reason why it should be removed in future versions. It would be like releasing games on floppies when CDroms are a standard.
Check the "use VDP1/2 Draw/CD Block thread" options. The checked part will then run in a different thread, so in theory SSF supports up to a quad cpu system.
You can also set the Slave CPU to 100% speed and the dot clock to higher for better compatibility, of course that will have a significant performance hit. Thats why a multicore CPU comes extremely handy.