Specs are like statistics...;-) Judging by the frequency response graph on the spec sheet, the sensitivity measurement is for output above 100Hz taken in free air, hardly subwoofer territory. The box dominates sensitivity for subwoofers. The 315HF is displacement limited to 102dB@20, 109dB@30 and 114dB@40Hz. It levels off around 117dB@50 with 500 watts, where you'll start finding the thermal limits before you reach the excursion limits.
A driver's a driver. The Iron Law for subs is low, loud, small...pick 2. PA oriented drivers, which appears to be what you're hinting at with the "consumer driver" comment, are biased towards loud and use relatively small cabinets for the driver size, but they don't need to get much below 40Hz...the lowest note on a standard 4 string bass is at 42Hz, kick drum fundamentals are typically in the 50-60Hz range. There is no free lunch although high excursion drivers with gobs of power can give that impression.
And I agree, one should be enough to keep up with the mains into hearing damage territory in a properly calibrated system. However, Matt has expressed the desire to ultimately have 4 so leaving unused output in the first pair due to underpowering wouldn't appear to meet his design goals. Not to mention, the IPR @ $299 is considerably cheaper than the SA1000 @ $400ish while providing twice the power. His goal could also be to get single digit response, which to be felt, needs gobs of displacement, EQ, and power even if the >20Hz output is kept at OSHA approved levels.
-Brent