Brian,
Yes, 6 dB if you double the speaker AND power. And yes, the diminishing return comment is accurate especially with a limited voltage supply. SPL challenges are for short bursts only and the serious SPL vehicles would not make it as a daily driver when used that way.

MicroSub...Nice little sub in its day. A bass-reflex has a steep rolloff below the tuning frequency and a smaller 8-inch would obviously be tuned a little higher. So the crossover frequency would need to be at least an octave higher than the tuning frequency in order to avoid a narrow and peaky response. It was probably crossed over too low and gained too hot. Plus a small bass-reflex sub really requires a subsonic filter to protect it below the tuning frequency where it acoustically unloads. That is another reason why you saw all the extra and wasted excursion.

Later on the other questions.
David
Earmark Marine