Tuning a box lower while making it smaller can help extend the low end a little more, but in the end you always get less low frequency extension. The only thing that differs it from a high tuned ported box is there is no response peak and the response rolls off shallow down to Fb, where it then rolls off.

And the answer is no, running two drivers isobarik will not give you any effeciency increase.

If you are planning on running your woofers isobarik on any conventional Pi speakers, they won't work unless you shrink the box volume appropriately and thus have an extremely long port.

Also, it's not a great idea for Pi speakers because the woofer is always called upon to produce the lower midrange band. This is not good for isobarik setups, and clamshell in particular.

Adam