Huh, I never really thought of that, but it really doesn't get stronger. It seems odd that returning the power to the Master Sword wouldn't make it actually deal more damage. The reason Link needed to restore power to the Master Sword was to defeat Ganondorf. If my memory is correct, Ganondorf himself had said that the Master Sword Link had at the time (before the power ups) was depleted of its former power. So, in a way, it did make the Master Sword more powerful, but not against regular enemies.
(TP Spoilers warning)
You could actually compare this to Twilight Princess, when Link retrieves the Sol Spheres and the Master Sword is infused with their light. I had originally thought that because of that, the Master Sword would be more powerful against the enemies in the Twilight. I guess these things were just plot devices/game mechanics rather than an actual upgrade.