My 12 ga. lid figures to be right at 150 pounds, by my dimensions. Then adding the cargo rack, and then possible a RTT, and it would be easy to hit 300 pounds. This amount of weight on the struts doesn't really bother me, its having this much weight at the top of my trailer that I wish I didn't have.
At the moment, I am planning on using 4 gas struts, 2 per side. Even though I can get 2 struts to do the job, gas struts come in different sizes, up to 250 pounds per Strut. I just think I would feel a little safer with this much weight over top of me while I am trying to put my gear in and out.
Its not the struts themselves that worry me, its the way that they mount. I have had mounts fail on hatch back lids before, and they come down pretty dang hard.
What is the dimensions of your lid going to be?