No. Abilities are lost the turn they change. So on the turn Ampharos gains its Mega ability, it will lose the boost from Plus.
>Changes in a Pokémon's Ability will also take place immediately. A Sharpedo with Speed Boost will not gain a boost in speed at the end of the turn it Mega Evolves, but if it uses a biting move such as Crunch that turn, it will be boosted by Strong Jaw, Mega Sharpedo's Ability.
The only exception to this is an ability that determines speed, e.g. Banette will not go first when using a non-damaging move on the turn it mega-evolves and gains Prankster, only on the turn after.