I’ve watched a few videos on using Monster Power for leveling. The mathematics essentially demonstrate the efficiency of using say MP1 as opposed to MP4 in terms of XP/hour. The justification is that higher monsters have greatly increased health, which takes longer to kill as opposed to the calculated amount of experience.
So my theory is that if you’re at a lower Paragon level and/or have poor gear, it’s probably best to stick with MP0 or MP1 at most for chasing after Paragon levels.
If that’s the case, where does higher level Monster Powers play an effective role in the game? In my estimate, gear farming is where high MP shines. Since Monster Power can exceed the normal Magic Finding cap (along the Nephlam Valor), you will want to employ higher Monster Power levels for gearing to really maximize your findings. I think the idea is that (unless you’re an exceptionally skilled player), you want to focus on surviving higher MP levels over itemizing your gear for Magic Find. Of course, if you are skilled and are able to swap gear on the fly, you’ll get even more out of this method.
Another interesting thing I’ve read is how many players have had little luck in finding good drops (namely legendaries and set gear). I’ve managed to find a couple of pieces on occasion that was a result of using higher Monster Power. My natural Magic Find is quite low without Nephlam Valor stacks so typically I find garbage. Being at Paragon level 11 at the moment helps only a bit. However, I think the sweet spot where people start finding better stuff is around Paragon level 33, where you gain a natural 100% to Magic Find in combination with Monster Power and Nephlam Stacks. So it might be worth focusing for a while on leveling to around the mid to upper 20s before trying to tackle gear farming on higher monster power levels.
Of course, this is all subjective and depends on your goals in the game. The way I look at it is just the course of building up to the point where you can eventually make a grab for the best game items. Perhaps an interesting strategy is to break down MP by every 10 Paragon levels. You don’t have to do it this way, but it’s one way to look at handling Monster Power if you don’t have the budget to get better gear initially.