they're two completely different models of subs, with completely different designs... and meant for different purposes. so you really can't compare them by saying that an 8" of one model has ALMOST as much power handling as the 15" of another model...
That is the same as saying 2 subs from 2 different brands made with different technology and design can't be compared. In this case, other than the number of voice coils and increased power handling, there are not drastical differences between the W0 and the W3 series.
They share the same purpose: bass reproduction. They have different applications, based on the frecuencies they were designed for.
The only reason to mention them was to point that a small and a huge sub can handle the same amount of power, and nothing else.
the reason a bigger subwoofer usually has higher power-handling than a smaller sub is not because of the frequency it's designed for... it's because the larger the sub, the more power it requires to move. however (as you mentioned), it IS designed to hit lower frequencies (the larger subs i mean) but lower frequencies aren't what makes it require more power. moving the sub requires more power ,since it's big
Let me put it this way: There are 15s rated at 125W RMS, there are 8s rated at 125W RMS. That means that both diameters require 125W RMS to fully accomplish their purpose. However, an 8" will not be able to reproduce the same frecuencies the 15" does.
Lower frecuencies move greater volumes of air. That is what requires the power, not the size for itself. Size makes possible to move those amounts of air in a better way.
If size was the determinant factor for power handing, there would be no point in making an 8" rated at 50 RMS and another rated at 125, or a 15 rated at 150 and another rated at 1000.
More RMS will only make a sub to reproduce the same frecuencies louder than another of the same size with less power handling, assuming they are similar in all other aspects.