Quick question; does it change anything to put Lucky Egg on your lead and use O-Power EXP boost in order to level with EXP Share, if your lead Pokemon (who will be the only one battling) is level 100?
These are the rules of EXP in Gen 6, for each individual opposing pokemon.
(1) Each pokemon that participated (which means it was present on the battlefield at any time) receives 100% of the EXP.
(2) If EXP Share is on, then each non-participating pokemon receives 50% of the EXP.
(3) If the opposing pokemon is owned by a Trainer, then all pokemon that receive EXP gain a x1.5 modifier.
(4) If EXP O-Power level 1/2/3 is active, then all pokemon that receive EXP gain a x1.2/1.5/2.0 modifier.
(5) All additional EXP modifiers are determined for each individual pokemon and modifies only that pokemon's EXP received. Examples of modifiers are Lucky Egg (x1.5) and being a traded pokemon (x1.5).
All EXP modifiers stack multiplicatively.
(There are two additional EXP modifiers that are important to take advantage of if you are power leveling in the Restaurant battles, and they were both introduced in Gen 6. (1) A pokemon with affection level 2 or higher from Pokemon-Amie gets x1.2 modifier to EXP. (2) If a pokemon is at or beyond the level at which it may evolve and has not yet evolved it receives a x1.2 modifier to EXP.)
(Note: In Gen 5, EXP is also modified based on the level difference between the opposing and player pokemon. In all previous generations, EXP is also shared evenly between all pokemon eligible to receive EXP. Gen 6 EXP rules are much more simplified and it has become dramatically easier to level to 100.)
.... in short... giving Lucky Egg to a level 100 pokemon does nothing.
Just for funsies:
The largest possible EXP modifier is x1.5*2.0*1.5*1.5*1.2*1.2=x9.72, for a traded, level 2 affection pokemon that can evolve but has not yet, which holds Lucky Egg and that participates against a Trainer's pokemon, while EXP O-Power level 3 is active.
Actual experience gained is EXP = MOD*BASE*LEVEL/7, where MOD=modifers, BASE=base EXP given by pokemon species, L=level of fainted pokemon. This formula is cut in half for any pokemon that receive EXP solely through EXP Share.
Blissey has the highest BASE with BASE=608, but cannot be found at high levels among Trainers in Gen 6. Audino has the third-highest BASE (after Chansey) with BASE=390. It can be fought in Restaurant Le Wow against a Trainer at level 63. So the highest possible EXP gain from a single pokemon is with a x9.72 player pokemon against this Audino, to give a whopping total of 34,117 EXP.