Yes the transmission is all about aerodynamics, and it's also the second heaviest mechanical part of the car, so also influences COG. That's why the transmission is such an important part of the car. Like you said, it opens up room in the diffuser, and since it's bolted to the engine, and the engine is bolted to the chassis, the 3 form one stressed member, and all 3 are responsible for COG.
So transmission and power unit layout both are integral to aero not just because of the heavy lumps, but because of the all the ancillaries, all the heat exchangers, the fuel tank itself can weigh as much as 130kg filled to the brim and affects the balance of the car as well. All of these pieces are in front of the most critical aero real estate. Which is why aero is more than just the stuff you see on the surface of the car, but trimming the fat from the stuff under hood is also aero improvements.
Something as innocuous as redesigning the wiring loom can shrink wrap the chassis 4.2mm in an area near the side pod, which lets you put in a duct that lets you feed a radiator more efficiently, which lets you reduce the size of the side pod opening to cool this radiator, which gives you a 2 cm tighter engine cover. Which then diverts a little more airflow to the rear wing retraction, which improves the performance of the rear brake winglets.
The slow road of R&D is slow because there are so many knock on effects which are dependent on the chassis itself. You can have very good aero pieces, but if the stuff under the skin is too big, too heavy, occupies too much space, then it will never work as other designs.
Maybe the big spending in 2020 for 2021 comes from going as extreme as possible with the transmission as allowed. 2021 will see transmissions homologated for 3 years, with only modifications allowed to "reduce costs" every other year. Knowing this, how will the Renault power unit layout help or hinder their transmission design?
Ferrari is rumored to be making big architecture changes revolving around shortening the transmission, what are the odds the big change is a split turbo setup? How would Renault make the transition, especially if the split turbo is the layout that gives you the best transmission design? Surely it would be important for 2021 regulations that have very prescriptive body work rules.
Increasing aero real estate by having a better transmission seems the only way to get a big advantage.
The test benches are very important because they let you keep developing the transmission, you can stress test them until they fail, see what works, where you can go more extreme.
Renault, McLaren, Mercedes, Ferrari, Red Bull and Williams are the only teams that make their own transmissions.
Alfa, Racing Point, AT, Haas have to rely on their suppliers, Williams may go down this route. Clearly the well funded teams can make a better chassis, and a better transmission to go with it.