It really depends on 1) mounting position closed, 2) shock strength, 3) position of shock with the lid open. You need assistance opening the door, but not so much it will not easily close, so there are two cases to consider.
Not all installs are the same. The back window of most SUVs has a strut, but it is to help hold the window open, not for assist in opening the window (though it typically does that also).
If your mounting position is nearly level with the lid, you won't gain much benefit compared to the rating (maybe 20%). For a good hint at the engineering principles, just look at the AT Chaser positioning of the gas spring with a box closed, and then opened. Notice the spring extends, but does not really rotate a bunch? Also, notice that the spring gets LONGER as the top rotates open.
BTW - There is actually an incorrect way to mount them, that will result in 0 assistance. That would be to mount them nearly parallel to the lid, mounted at the hinge and the midpoint of the lid edges. Mounted this way, the spring cannot expand to assist with the lid and really has no value.
I will be looking at this in detail this week. I need to figure out where to mount them in my M100 to get the most bang without hogging packing space near the tailgate.