I have 6 plants (all around 4ft tall or so) under a 600 w HPS. They are in they're 5th week of flowering. Im thinking about adding 2 - 4 ft T5 lights (54 W, 5,000 lumens each) to add lighting to mostly underneath where theres not much light. Im doing this to hopefully increase yield (obviously and because ive had a bit of a powdery mildew problem, although its improving because of an added fan. I just figured there some people that can give their 2 cents on this as im a noob - only have done autoflower before.

Thanks!!

Side lighting is great man, especially if you are noticing problems with light getting to the lower parts of your big plant. I'd go with regular CFL's if you don't already have the shop lights, since they should be cheaper and will certainly be simpler to position, since you could hang and stuff them into nearly any position. Its very difficult to cause burn with CFLs in my experience, so you can really get them in there.
If you do have the shoplights, then go for it. No point in spending more money if you already have something that works.