Rather than just tell you that you're wrong, lets take a minute here and explain this. You need to look at the total linear displacement of both. And, you can't just say "double 8 and you have 16, so two 8's = one 16". You have to look at the actual surface area of the cone (Sd) and the linear excursion of each subwoofer (Xmax) to determine the total linear displacement of each subwoofer(s).
Lets take a look at the 8w7. It has a cone surface area (Sd) of 245sq.cm. and an Xmax of 1.9cm. Multiply the two to find total displacement. 245sq.cm x 1.9cm = 465.5. And, since we would have two of the 8w7's, we can multiply that by 2 for 931.
Now, the 13w7 has an Sd of 639sq.cm and an Xmax of 3.2cm. Multiply the two and you get 639sq.cm x 3.2cm = 2044.
As you can see, a single 13w7 has more than twice the linear displacement of two 8w7's.
