My intent is that preset ones would ultimately be be no more powerful than optimized custom ones. It's just that the custom ones would benefit from mechanical synergies (found by a skilled player), while the preset ones would have some special features not available otherwise. Or perhaps just have more stuff that's flavorful, but not as useful.
So if the custom build rules allow for, say, three feats at level 6, the pregens might have 4 or 5 feats, but they'd be less synergistic than what a custom build could go for. But if someone liked those specific feats, they might pick it.
Like I said, the Essentials Fighters have stuff that the "custom" 4E PHB Fighter can't get. But they're not better, and plenty of people would favor one or the other. Similar principle. You can build pregens that break the rules of the custom builds, and arrive at a balanced system.