Enemies EXP caps at +10 levels, to increase SP/hr solo kill stuff only 8-12 levels higher than you instead of 15. Kills should be twice as fast and the mobs that are 10-12 higher will give the exact same as the ones you are killing 15 levels higher assuming they have the same base SP.

If you are getting 150 SP per kill at +15 that means the mob base is 50, which isn't a mob you'd really want to fight for SP. You might want to find a different mob that has a base of 80-100 so that you'd gain 240-300 per kill.

Also mob SP degrades fairly slowly once its about the same level as you. L43 killing L42 doblyn's will still net you 90 per kill or so, which would probably be leagues quicker than fighting L54s for 150 since you'd kill about 3 in the same time + lose less HP/MP.