I disagree. In the real world, you typically have a room that you can only do minor modification to if any, and you have already purchased the subs whose ground plane output is already a pre-defined quantity. Unless Seaton talked you into buying the 12 pack of Terraform XL's, you are probably more concerned with what your MAX output capability is, at the seats. This will vary a bit by frequency, and will vary with sub placement.
Assuming the placement is done, you can either level match or gain match the subs. In the gain match approach, both hit their max output at all frequencies at the same time. This is also the max output you can get from your subs in their current location.
With the level match approach, one sub has to be turned down so its level matches the level of the other one at max. In this case the gain increase from running both subs at the same time will be 6 db, but since you turned one down, the 6 db is added to the lower max output, and the summed output will be lower than in the gain match approach.
Anyway, what I am saying is what matters is what you can get for max peaks under each approach. With decent subs turned down well below their limits (75 db for example) you will not notice any difference between the approaches because you should have gobs of headroom to spare (or you have two wimpy subs and you should give Mr. Seaton a call!)