Jesus ****ing christ. It's still relevant!!! Flat is Flat, Room is Room. The ground plane measurements are all the same because it's the same person and the same gear. Now, take those ground plane measurements, and put them in the same room as the other subwoofers measured by the same person, and you have a relevant comparison. Here, I'll spell this out for you, step by step.
Sub 1 measured ground plane. Sub 1 has a plotted response ground plane. Sub 1 is measured in room. Sub 1 has a plotted in room.
Now do this, for every sub! You can see the gains relevant to the ground plane measurement when used in a room, and when you make this normal for every subwoofer, you can see how they benefit to room gain RELEVANT to their ground plain response. Sometimes you don't know HOW MUCH IT WILL CHANGE when used in room, so measure both and SEE!
Obviously the in room won't be the same as yours, just like ground plain, unless you live in a trailer park. What we have is in room responses, we don't listen ground plane. So knowing what will/can change is better. More information is ALWAYS better.