If it is overpowered, it is the math that needs fixing, and not the ability to split your resources however you want.
Have a banana. That's the nub of the issue.
See, the problem is, the math is still at fault. And so we can still produce insane output numbers.
Let's say I decide I'm sick of wasted production. So I'm going to make most of my planets industry and keep the global slider at 75% manu(so that, with focuses, I get 100% output on them). I set econ to zero and research to 25%.
Bang, I'm straight back at producing a superdreddy every turn from every industry world. Sure, my econ output is low, as every econ world can only work at 1/4 efficiency, but since money is still pretty easy to come by I can live with that. And yes, my research world operate at half-power... but that's not much below what they're be getting if I was splitting global production three ways anyway, and my TOTAL output is vastly higher. I am still perfectly capable of cranking output up into insane levels. It's worth noting that I'm also still being just as coercive as ever before; I'm just doing so at an empire-wide level. Finally, it should be noted that this is EXACTLY why the whole all-factory vs all-lab thing emerged in GC2 - because the perverse incentives that the system introduced meant that mathematically, the only way to minimize wasted production was to specialize the whole empire rather than specializing individual planets. Rather than encouraging us to produce individual planets, it encouraged complete uniformity, even more so than the wheel did.
Moreover, the problems with production were actually twofold. There's problem 1, which was that production numbers were completely out of scale with costings. Focuses fix that, unless I'm willing to sacrifice a resource, in which case as we've seen they don't; even if I just drop econ and set manu/res to 50/50, a 25% reduction in output isn't enough to deal with the difference.
But there's also problem 2, which is the difference in output between a full-factory planet and an otherwise identical no-factory planet. This is actually a bigger problem than the cost-output imbalance, since the relationship between them determines what 'massive over-production' really is. And the whole focuses vs wheel thing doesn't impact on it at all. Allow me to explain.
It is a necessity that the cost of buildings is largely tied to the low end of manufacturing. Even planets which have no industry need to be able to build things in a reasonable amount of time. So our costings are, by and large, capped at a point where any individual item cannot realistically take more than 10 or 20 turns or so for a planet with only 1 or 2 factories to build.
Now, an all-factory planet will produce some multiple of the non-factory planet's output. The size of this multiple is thus crucial; it should be maybe 4 or 5 times as high. Once we start getting to 10 or 15 times as high, we reach a point where it takes a very long time for a non-factory world to produce something which the factory world can still churn out in 1 turn.
Changing from the wheel to focuses doesn't alter that multiple. If you take a world with no factories and set it to manu focus, it's manufacturing ratio to an otherwise identical all-factory world is exactly the same now as it was with both just set to 100% manu on the wheel. Thus, if balance is adjusted to make low-manu worlds capable of building properly, then we'll end up in exactly the same boat as before with massive overproduction from specialized planets. The actual numbers themselves are pretty much irrelevant, since all that really matters is the number of turns it would take a planet to build a structure with no factories, and the number of turns it would take it to build the same structure if it only had factories. If that ratio is over 5:1 or 6:1, then nothing gets fixed, as we either end up with non-factory worlds being incapable of developing, or factory worlds being massively over-productive.
And that's really the point. The reason all the explanations given for swapping the wheel for focuses are so unsatisfying is because focuses are themselves pretty unsatisfying for dealing with any of them. We were told it would reduce micro; it does, to an extent, because MM is less worthwhile now, but it's not as effect as giving us proper macro-management controls would have been. We were told is would help with balance; and it does, to an extent, but not as effectively as actually balancing the output and costs properly would have been. We were told is would make governing feel less coercive... and it doesn't really do that, because as many others have said government is still directing what everyone does, it's just doing so more incompetently now.