I recently switched from viparspectra and picked up 7 550's in September. Just completed harvest with EXCEPTIONAL results. Many other variables (different and much larger space, environment, etc) but the controller is the real key. You won't be running these at full blast unless you have a need for the heat from the whites. It was a first run, and I maxed at 4 lights each covering 2'x3'. Ideally, I will have 6 each covering 2'x4'. This would leave me with an extra panel in the event some maintenance was necessary.
Not an apples to apples comparison, but:
ViparR900(1)/NirvanaAF/Air-pot #2/Canna coco & nutes
213g (.53gpw/287g/m2) from a 4x2 space in 63 days (forced harvest 3-4 weeks early). The r900 is ~400w actual.
Three strains tested @ 17.9%, 18.5% & 18.5%
SS550(4)/FastBudsAF/Air-pot#5/Canna coco/ANutrients
808.74g(1.01gpw/363g/m2) from 6x4 in 74 days. This grow had some issues an was less than ideal. The AN is simple enough, I just don't feel that the plants ever looked as good as they did on Canna. AN was returned under the growers guarantee after 3 months (and they give you 6 months) and I restocked Canna. Test results should be up next week.
The difference between the two makes it difficult, but the spectrum controller is where it's at. I had one light for the first 2 weeks, 2 for the following 2 weeks and then 4 lights for the remainder. I had roughly 8 steps programmed, rarely using whites (10-30%), reds (10-40% veg/70-99% flower) and blues (70-99% veg/10-40% flower).
I adjusted the g/w to reflect each panel running at ~50% draw. Had unexpected heat issues in room and the red/blue give off very little. I used whites mostly in midday, but not much early or late in the day.
Sidenote: FastBuds have some sketchy genetics & I won't be running more anytime soon