I still don't see whats actually wrong with Mirage. You can use his decoys every few seconds, which is a plus already, to have the enemy look one way while you go the other.
His decoys are meant as a quick distraction, to draw gun fire and fool the enemy while you run or get behind them for the finishing blow.
If his decoys were to be able to show hit markers (even while not actually absorbing the shot), than it becomes a whole different thing.
Imagine being in a gunfight, you see mirage running, you fire, you continue to fire because you see hit points but after 6 seconds it disappears and you realize you left your team fighting a 2v3. That doesn't sound so fair or fun to be the receiving end of.
You could ignore the decoy but what if it turns out to be the real thing, now you screwed yourself and the team. Too much power for someone who already goes invisible.
Where a buff is needed in my opinion is having the ult last a few seconds longer to fully get behind a team or run away.