Personally, I think they should balance SP across the board by level, and not by mob type. A level 30 will give you nearly the same exp (maybe with some random small digit modifier to make OCD people rage), and/or a mob that is the same level as you. The former system is a static system that nearly all MMOs use; the latter is the system FFXI used in which xp scaled to your level. Personally, it's a system I wish they used more often, as it makes the grind feel more linear, and essentially promotes trying to kill mobs that are intrinsically more difficult for the greater exp benefit they offer.

But seriously, balance the damn SP rewards between mobs. If I see a vast field of rabbits that I can slaughter, I shouldn't be penalized just because they're not coblyns/raptors/efts. I want to KILL DA WABBIT! ...and get great SP while doing it.