If a same-size-as-the-M2 loudspeaker was OPTIMIZED for one octave less low-end extension (including a different woofer or woofers with very different parameters), theoretically you could gain as much as 9 dB of efficiency. In practice you'd probably need to use two woofers and either very aggressively EQ the high frequency section or completely re-design it, probably trading off radiation pattern width for efficiency within the coverage angle.
Obviously this is not the ONLY way to increase efficiency over the M2, it's just one example, meant to illustrate how much efficiency you can theoretically gain by giving up an octave of bass extension.
This might be a more practical example, a (passive) loudspeaker with 3 dB more voltage sensitivity than the M2:
PBN M2!5. This probably illustrates the upper limit of voltage sensitivity we can squeeze out of the M2's driver/waveguide combination before needing significant active EQ to boost the top end.
(Looking through the other end of the spyglass, my first example illustrates that extending the bass one octave deeper without increasing the enclosure size will cost you about 9 dB of efficiency. This is why small boxes that go deep are not very efficient. Or, if we DO keep the efficiency the same and extend the bass one octave deeper, we will need
eight times as much internal volume, assuming optimized woofer parameters. Tradeoffs, always tradeoffs.)