Likely it would be a good idea to have all the information before 'guessing' at the causes and remedies. Fill-out the proper troubleshooting form in my signature (copy-n-paste) to the best of your ability and knowledge.
However:
I use one 1000w HPS for up to 12 plants in 5 gallon pots. I can fit up to 15, but they need elbow room once they fill-in, and more room makes it easier to water the plants in the back of the growspace.

There is a point of diminishing returns. Basically, you can keep adding light, but at some point the plant won't be able to handle the excess light and heat, and has to start protecting itself from the assault. I'm guessing you've exceeded it's limits, but it's a guess.

I've also seen hydrogen peroxide treatments and fried roots 'canoe' the leaves. Fill-out the form.

Are the canoed leaves leathery or crispy? How long do they take to become crispy?