Will these hold up in the heat of an average attic in the mid Atlantic. Or are they prone to sagging?

Not. If you're planning on putting them in a c-van or other metal container that would be subjected to over 100° heat! I have bought these and some other brand names that are extremely highly rated and put them in a c-man where it was ventilated and not overloaded and they did get a little weak in the summertime probably would have been best to have them anchored to something but I did not have that option. They do warp with time no matter what. They are still plastic

This shelving unit is rated to hold 200 lbs. per shelf when weight is distributed evenly. For maximum stability, please be sure the support poles are fully seated, adjustable feet are leveled, and unit is secured to a wall using the included brackets. For every 5°F above 95°F decrease weight on shelves by 10%.