I'm sure you could get away with fewer drivers if ran parallel strings, etc, but yeah the cost effectiveness goes way down. However, I don't think you would ever be able to get the efficiency and nutrient uptake with any other method of lighting, so it is what it is.
As far as going 60 per side and only one screen increasing nutrient uptake, I can't say "Yes" definitively, because you are feeding such a huge amount. I would have no past example to compare it to. But if you took all of the flow you are currently using and put it on one screen instead of 2, and put all the light on that one screen, I would think you would get overall equal or better production of algae. All you can do is try, and if it's not enough, fire up the second screen again. But you need to give it enough time to really get growing thick and fast. You will probably want to clean the screen one side at a time every 3-4 days and keep the slot clear and flowing well.
IMO once the screen is mature, and if you have heavy feeding (at least equal to the screen capacity as defined by the feeding based guideline), you can crank up the flow as much as you can, and blast the screen with as much light as you can.