The Reptisun T8 UVB tube should be a minimum of 8" from your beardie, unobstructed by any type of mesh, glass, or anything else. The T5 is a minimum of 11".
As far as hornworms go, I can't really help you out, as I've only fed them as an occasional treat, and the most any of my guys have eaten at one time was 2-3. Are you feeding them as a staple, or as a treat? They are very filling, and a lot of this depends on what size hornworms you're feeding them, because a large hornworm can fill up a small to medium sized beardie. Hopefully someone else with experience feeding hornworms on a regular basis can help.