http://medicalmarijuanagrowing.blogspot.com/
Something to get you started, second post in there. That talks about the 5% increase in thc they saw from it. I will find the yield things later.
Alot more testing is coming out I seen geared towards this. To me it makes sense I don't know. If a plant get hurt it will focus on repairing it's self. If it's growing bud it will focus on growing that bud, thats why we want perfect tempatures and no mishaps Ie something falling on the plant. Because it stops to repair.
So with all that to me it was logical that it may stop producing bud to shield its self from the light. I have used UVBs and I saw mine oozing before. Could of been the strain (Ghost train haze) but it was amazing to see.
I remember reading it wasn't a big drop maybe an ounce or something. But to increase personal smoke it's worth it of course imo. But just something to think about.
Now I am not 10000% sure how a HPS with no glass compares to a stright up T8 (uv rays) light. I know if you remove the glass, you are now introducing uv lights to the grow as it gets filtered through the glass.