This post is actually going to be fairly short because it's about 10:00PM right now and I've got some stuff to do. It's not going to go over what I said it was in the previous post.
The first thing that needs to be done is to find averages of everything. Average total mana, average cast speed (we do not need to balance the game based on maximum cast speed, reaching the max should give a large bonus), and what the average cost for a spell is all at an average level.
At some point we have to accept that our characters are godly unkillable murder machines and that attempting to balance the game around them is counter-productive to what balance in an action-rpg is all about.
That should be fairly easy. We don't need to examine each and every spell, just the most used spells.
We should record the average total mana and cast speed of a hybrid as well.
Once we have this data, we can simply take the information about potions in the art manager and compare them to what exists. Then we can truly assess if the problem does indeed dwell in the realm of mana potions.
After that, the change could be as simple as increasing the re-use time of energy potions by X amount of time based around average mana consumption at Y level, going through each type of mana potion and giving them different reuse times based on mana given to the ratio of mana spent per second, or making it so mana potions can only be used once a minute so they are used in emergencies.
One flaw with the out of combat potions is that it negates a large part of passive mana and health regeneration. While they are constantly on, you get the most benefit of them while you are not in combat. Loosely restricting these potions to out of combat only might make certain energy related things matter more, but they'll make others matter less and that is not a good solution.
The best solution would be the Diablo 2 belt method mixed with the TQ quantity method coupled with time restrictions based on the type of potion you used. You could have more in-combat oriented potions that give smaller bonuses but give smaller reuse bonuses and larger potions that are primarily used out of combat or in emergencies which have larger reuse penalties but that doesn't seem to be a realistic solution.