Vortex,
I think we are missing the point of what a subwoofer does. A sub is meant to add low-freq weight to the whole audio experience. It is normally meant to be felt, not explicitly heard. Ideally, subs carry forward the audio spectrum after your main speakers have reached down to their lowest level. In a good floorstander, this level is around the 30-40Hz mark.
To cite one example, both REL and Quad L have comparably large drivers but are among the best in their price class. I think what you're referring to as far as driver size is concerned is when you use a typical sub-sat system, where the sub does work starting from the lower midrange to the low frequencies. But then, this is not really a subwoofer, just a woofer in a separate cabinet. In that case, you're right...larger driver-> more inertia->slower transient response. However, the classical definition of a subwoofer is one that extends the low-freq reach of the main woofers.
Ajinkya, the frequencies that would only be felt and not heard start from 20 hz onwards. Seriously speaking there are not too many woofers in the price range that Pradski or Anooj are looking for that would go so low.
And while it is important for the subwoofer to go lower than the fronts, it also depends on how it presents the low. Whether it is loose whump or tight slam. If the bass is tight, often times I can make do even with lack of depth in frequency. But that is a personal preference. Some people like echoing, decaying bass notes. There is one for everyone, I guess.
In Pradski's case, the subwoofer also has to be one that goes significantly low than 28hz which is how low his Diamond 9.6's go. And that, as you know, is pretty significantly low for a standmounter.