One study did show that UV helps in the conversion of CBD (another cannabinoid) to THC using light with a wavelength of 235-285nm. Raphael Mechoulan was the scientist and the source of this information is in "Marijuana Botany" pg 132-133. I believe that this is the origin to the claim that UV can be benefitial to the plants, however, this study was done on a solution of CBD and not a live plant.
I'm not aware of any study of UV light on live pot plants so any answer that you get should be treated as speculation.