In a group we have here in Argentina, they say that both the RR and SP have nikasil. They can have a steel liner if they were repaired. Yes, the SP has more agresive ports.

For what I could guess, I would say the SP is some sort of factory tuned version of the RR, something a little better. Maybe they designed the SP, but they still had a lot of older model parts to build a lot of RRs, so they sold both versions at the same time... I really don't know.