8 ohm speaker rated at 88dB will produce 88dB/W at 1m distance. At 2m it will drop to 82dB/W, but room reflections add 3dB and second speaker adds another 3dB, resulting in 88dB at 1W. At 23W they will produce 102dB - should be loud enough.
Sensitivity is specified at 1W or 2.83Vrms. It is the same with 8ohm speaker, but specifying 4ohm speaker at 2.83V (2W) shows better (by 3dB) sensitivity in specs.
Speaker's nominal impedance is specified usually at 1kHz, but might vary a lot at different frequencies. As you stated lower impedance speaker is a little harder to drive, but the same goes for speakers that have impedance dip at certain frequencies. My amplifier is specified for >2ohm speakers, while my 6ohm speakers' minimum impedance is 3.6ohm - so I'm OK.
Sensitivity question
I need a little help in trying to understand some basic concepts with speaker sensitivity. I understand that an 8 Ohm speaker rated at 88db (or less) can be a little hard to drive---and 6 Ohm can be harder. I get that----but on some small British speakers, they can be 86db but rated at 10 Ohm. So my question is this: is a 86db, 10 Ohm speaker hard to drive or not? Can I run them with a 23 wpc Class A amp? Thanks in advance.