Opponent has Sage of Ancient Lore in play. Power is at 6.

On my turn, I cast Mob Rule, choosing all creatures with power 4 or greater.

Spell resolves, I gain control of opponents Sage. Sage's power drops to 3 due to it's static ability. Mob Rule's untap occurs, but the Sage does not gain haste.

Per official Magic ruling:
2014-11-24: Once you gain control of a creature, it doesn’t matter what happens to its power.