I'm going to stuff 14 boards into my 4x4 so I've been wondering myself about mounting them vertically.
For me it would just be to get more light to the outside plants and also provide another angle of lighting to the rest of them = better overall use of the photons. But I don't know how soft I'd have to run them.
I have a panel based on the same idea, from a couple years ago. A few thousand 1/4 watt leds mounted on boards covering 8x46 inches for about 200 total watts. That panel was rated at 120 Lu/W if I remember correctly and I could run it within about a foot of the canopy. My boards will be over 180 Lu/W, equivalent to about 33 watts vs 50 watts of the older design.
It seems doable if you can justify running them that soft (or softer).