Blurple lights work OK, not great. I could get about .5-.6 grams per watt used off of a GalaxyHydro 300 blurple. After changing my light from blurple to cob it's closer to 1.0 grams per watt used.
Yep, is also a great technique to let the Kushes strains stay short with tighter internodes...
Like in their homeland (Afghanistan, hindu kush) you see them shorter, bushier and with much bigger buds... that's because in the soil under just 10-30 cm there are rocks...
Not much people knows it, a very useful trick to avoid stretch in kush genetics.