Surrounding a t3 factory with t3 fabs is a terrible waste of their adjacency potential, even if you are getting effectively 'free' units late game you have much easier eco options such as mass fab templates or especially RAS com spam which doesn't require destroying and reclaiming your own units!
Using a grid to make units is efficient but once multiple t4s are on the field then t3 armies start to struggle.
A much more interesting use of t3 fabs would be early in the t3 phase to scale your unit production harder, using the opposite layout with a single fab surrounded by factories. Making percies or bricks the fab would save nearly 20 mass total a second, in addition to the generated 16 you are net 36 mass, which is two t3 mex upgrades!
Admittedly you need to power the fab and it probly wants a t2 shield if your team doesn't have air. So need an early t3 pgen - or possibly ACU RAS could work.
You can even get funky and include mex adjaceny:
Here you are effectively getting three t3 units for the price of two. It is pretty rare to get a layout for four mexes, but three is pretty common and you can do two with any single mex.
Four storages would give 3 mass but here are they are saving nearly 10. Again this relies on you constantly building units to get the benefit, but it is not true that assisting the factory will reduce the adjacency - you will still be saving the same mass per tick than you would unassisted, it is just that you get more units in the time and so the mass saved per each one is less.
What will kill your efficiency is stalling mass, if you are the kind of player who stalls for half their total income at this point in the game you are going to lose a third of your adjacency bonus, and should probably just build the storages and t3 mexes instead