Per Al's remark, my understanding is that the sensitivity specification is usually for a 300hz or 1khz input signal, so a sub wouldn't affect that (either way).
Moreover, even if that weren't the case, I'd think that a properly designed speaker shouldn't be affected by filtering bass from its input signal. (Whether you filter by crossing to a subwoofer or by filtering the output of the source, for example, shouldn't be relevant. Either way, from the main speaker's point of view, you're just removing bass.)
A "flat" speaker system would take a wide band input signal and produce a flat output across its bandwidth. In a three way system, for example, it's probably true that the woofer will require more power to produce a given SPL than the mid-driver. However, the speaker system should be tuned so that both drivers produce the same SPL when fed the same signal. The output of the mid driver may need some attenuation (possibly within the crossover) to achieve that result, but flat system response requires it.
If that holds true, then adding a sub will increase overall system sensitivity if that sub is more sensitive than the main speaker or decrease system sensitivity if the main speaker is more sensitive (provided the same input signal is used). The biggest difference in the combined sub-speaker should be in maximum clean output capability, not increased sensitivity.
Also, bear in mind that the sensitivity spec is probably quoted for an anechoic environment and you will get room gain in your listening room.
Moreover, even if that weren't the case, I'd think that a properly designed speaker shouldn't be affected by filtering bass from its input signal. (Whether you filter by crossing to a subwoofer or by filtering the output of the source, for example, shouldn't be relevant. Either way, from the main speaker's point of view, you're just removing bass.)
A "flat" speaker system would take a wide band input signal and produce a flat output across its bandwidth. In a three way system, for example, it's probably true that the woofer will require more power to produce a given SPL than the mid-driver. However, the speaker system should be tuned so that both drivers produce the same SPL when fed the same signal. The output of the mid driver may need some attenuation (possibly within the crossover) to achieve that result, but flat system response requires it.
If that holds true, then adding a sub will increase overall system sensitivity if that sub is more sensitive than the main speaker or decrease system sensitivity if the main speaker is more sensitive (provided the same input signal is used). The biggest difference in the combined sub-speaker should be in maximum clean output capability, not increased sensitivity.
Also, bear in mind that the sensitivity spec is probably quoted for an anechoic environment and you will get room gain in your listening room.