Just as higher tier engineers were adjusted to be more efficient in build power, higher tier mass fabricators should also be more efficient for mass generation, especially given the unit cap.
There are a lot of downsides to this idea. For example, such a change would kill off second layer ringing T3 mexes with T2 mass fabs, an important part of making late game raiding more impactful. However, I feel that the benefits of not eating up unit cap outweigh these concerns.
To be clear, I am not talking about the unit cap from just 2nd ringing T3 mexes, but from late game mass income expansion, where it is most mass efficient to simply spam T2 mass fabs. The only real limiting factor, aside from unit cap, is space.
A strong argument for this change is consistency.
It doesn't make much intuitive sense that a T2 mass fab would be more efficient than a T3, and by a noticeable margin, too. Of course, it is not quite as simple as comparing raw mass cost and energy draw to mass generation. Space is a factor, for instance, but generally, T2 mass fabs are superior to T3.
It also fits with the progression of the game. You’ll always upgrade your mexes to T3 before turning to mass fabricators, and if you're at the T3 mex stage, you’ve almost certainly gotten to T3 engineers and can build T3 mass fabs.
With T2 mass fabs you gain access to them well before you want them, and by the time you want them, you've likely already gained access to T3 mass fabs.
The more I think about it, the less sense T2 mass fabs make when T3 mass fabs already fit into the game so well. This change would probably make T2 mass fabs almost entirely obsolete, but I don’t see that as a significant problem.