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No. There is literally no way you can distribute +100 to Mega Sableye's stats and have it actually be better with Magic Bounce than it was with Prankster.
From what they've said of its stat boosts (+Def, +SpA, -Spe) it's meant to be a bulky attacker now, which makes Magic Bounce pretty much useless, plus it gets a ridiculously shit offensive movepool; Power Gem, Shadow Ball, Hidden Power, Psychic, Snarl, Dark Pulse, Dazzling Gleam.
It will have 480 BST as a mega, the same as Mawile, but unlike Mawile it won't have one of the best abilities in the game, especially as you can't even switch it into hazards to bounce them back without first having it out for a turn to mega evolve.