I actually think this is the bigger issue. Players are too strong. I can solo rank 64 mobs with ease, how on earth are you going to balance party play when players are that strong? Fight rank 75-80 mobs in party and get 1200 SP per mob? Oh wait, mobs that high two shot you at 50, not the best party mob any more when AoE goes off. The game should be optimised around the +10 situation.
Anything over 50 should be a challenge to solo while parties aim to kill mobs in the upper 50s during their 40s. Solo SP is soon to become leve based anyway, even with current over powered characters it's just not worth grinding solo. But they could boost the SP given for a soloer fighting difficult mobs which are now closer to their own rank.