1. With only one fixture for that many buckets, being able to rotate the plants so they all get an equal share of the sweet spot can help.
2. My yields doubled when I stopped topping my plants and started using LST- tying the main stem and all the branches down horizontal. Topping slows down the growth, and unless you've got time to top, and then top, and then top again, I get way more main colas with LST than I ever did topping the plants, and I don't lose time.
Now, if I only had good lights......