I'm more thinking how it is going to go fitting the 18" tyre on a 19" rim - can't wait to hear how you go though - if so the 3.0 x 18" will be my next rear tyre.
Supermoto tyres (after all the street legal kit like mirrors, lights etc...) would be the single biggest upgrade for street riding - suspension will be fine, seat should be ok, chain should be ok if the noise isn't too bothersome.