Paragon "farming" 2.4

Blizzard has massively changed the mechanics of XP gain in 2.4, so the average player gets about as much as they used to, and 4 player XP farming teams get much less. This was largely accomplished by increasing the XP modifiers for higher GRifts and slashing XP bonuses on gear.

In light of this, there must be a new meta on grinding out Paragon levels. Is it more efficient, from a purely XP/hour standpoint, to:

a) run grifts that are 'x' levels less than your max, as long as you can complete them in less than 't' minutes (and what are the optimal bands for 'x' and 't'),

or

b) continue to push Grifts, even if you don't finish them in 15 minutes?

(Assume softcore, 95+% gear perfection w/ Caldesann's Despair on each piece, and that Grift keys are no object.)