xp bonus rate of return?
 

I thinkt he xp bonus rate of return diminishes over the course of leveling.. at what point would it become more efficient to solo the way you would on blue, instead of grouping with whoever and as many as possible?

never, your bad on blue and now on red but I guess +1

If you can charm (enc, nec on undead, druid on animals), or are a bard, it's still faster exp to solo. Otherwise group up.