The right 8 inch driver can go down even to 28Hz, but the excursion is too large and the size of the magnetic gap increases. Larger excursions cause midrange intermodulation distortion, and the large coil size increases inductance. At some point you will trade off bass response with midrange precision.
I however think 8" is the most dangerous size to work with in audio. It is in nether territory, being unusable above 1.5KHz due to off-axis beaming, and the diaphragm is too large to be able to maintain pistonic behaviour above 2KHz. A very stiff cone would lead to a lot of uncontrolled breakup (I assume at ~4KHz). If an 8incher is called on to do midrange duty, one needs a tweeter that can go down to ~1.5KHz, and these are neither easy to find nor cheap.
Not to say that there aren't good 8 inchers, but I'd rather step down to a 6.5" midwoofer for most designs, a good trade-off between bass and midrange precision, and most are usable all the way up to 2-2.5KHz without beaming. For those who require a masseuse, a sub or a built in 10" woofer can extend the response down to the last octave.
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!! Am sure it sounds awesome 
