Based on the information so far, Mega Evolutions occur during battle (presumably triggered by certain conditions) and require a specific item. In other words, until the Mega Evolution occurs, the Pokemon functions at lesser capacity, as the item will probably not be useful beyond allowing a Mega Evolution to occur. The conditions which trigger a Mega Evolution may also easily limit it; for example, if it is triggered by being at or below 50% health, the Pokemon may need to take measures against being OHKO'd such as running Substitute (limiting the coverage moves the Pokemon can run) and will be far more vulnerable to revenge kills. If the Mega Evolution is, instead, activated by using a turn, it really is no different from setting up (which would change nothing for Ubers really, as Lucario, Blaziken and Mewtwo have good set up moves already). Unless the conditions in which a Mega Evolution occurs are very easy to meet and it does not take a turn, I do not see them being in any way overpowered. The tier is full of Pokemon which become extremely threatening if given a free turn.