Like others have said, I would suspect heat stress first, maybe the leaves are curling upwards to conserve moisture. Have they been drinking more or less water than normal? Do the leaves have a bluish or grayish tint? Are the leaves dry feeling? How do the roots look?

As far as mag deficiency, it's hard to tell what color your leaves are in the pictures due to the HPS light and you don't have any pictures of the whole plant so I can't tell for sure. It's easy enough to test though. Pick up some epsom salts for a couple of bucks from any drug store, mix about 1 teaspoon per gallon of water, put it in a spray bottle, pick an inconspicuous spot on one of your plants that's displaying the symptoms, foliar spray that area once a day and see if that area picks up.

I don't know much about the Sensi series of nutes or what ratios to mix them at...isn't that like a 4-part? Anyways, look on the labels and see which part has the Mg and make sure you're giving the plant an appropraite amount.

Make sure it's not a heat problem first, though...good luck!