I was aware of the line pressure concerns when I upgraded my trans cooling system.
After first adding a B&M cooler that had 3/8" hose ports that matched the OEM 3/8" lines the line pressure did increase a couple of PSI.
A 3rd cooler or a larger cooler with 3/8" tubing would likely increase line pressures in either case because there is a longer path for the fluid to travel.
The 5R110 cooler works because it is built with 1/2" tubing to cool the monster 5R110, so even though the cooler is over twice as large as the OEM V10 cooler the line pressures are lower.
I used a couple of pressure gauges and adapters to measure the line pressures with the combinations of coolers I was trying out.
The final setup of the 5R110 cooler, B&M 70274 cooler (with 1/2" NPT x 1/2" hose fittings) and oil filter results in the trans cooler line pressure being a couple of PSI lower than the stock OEM V10 line pressure.

 

I'm impressed with the detail of your testing. Very interesting.