from a theory and a practical perspective, t3 mass fabs are completely outclassed by t2 fabs. There are 2 uses for mass fabs, ecoing, and adjacency. In both cases t2 is way better.
Eco
The energy costs of t3 fabs make it flat out worse than t2, except in the instances where you have low space/bp (in which you should probably get sacus. on their own, t2 fabs give 1 mass for every 150 energy while t3 fabs give 1 mass for 293. You might think that the greater suvivability and more compactness of the t3 fab lets them pair nicely with t3 gens to shave off some of the e costs, except that a lone t2 fab is still more e efficient than a t3 fab with one t3 pgen (1 mass for 236 e) or even two t3 pgens (1 mass for 182 e).
In addition, t2 mass fabricators pay off their mass cost in 100 seconds, while t3 take 250. they are also in my opinion far safer for ecoing as you can just build small groups of them in the corner of the map. their death explosion won't hit anything with a 2 square block gap in between them.
Adjacency
When it comes to adjacency bonus, it's a bit more even, but to even get close to balanced it requires a lot of setup. If we pretend that the percival is the only unit in the game, building a t3 fab next to a factory saves around 4 mass per sec. With a complete ring around one t3 fab it can actually become slightly more efficient than a t2 fabricator (granted this is without considering any adjacency for the t2 fab, and the fact that this special t3 fab still has pay back it's own cost). But doing this takes an extreme amount of setup, and runs the risk of losing all your production to 2 strat bombers (which are dirt cheap by the time you can setup). with t2 fabs you don't get such a big bonus (unless you ring a factory with them) but a single t2 fab is actually affordable mid game and won't blow up in your face.
ideas for balancing the mass fabs:
I'm more inclined to buff t3 rather than nerfing t2
reduce the e cost of mass fabs or increase their mass generation.
increase adjacency bonus and maybe hp.