Unless you want to replace the Yamaha speakers, I wouldn't worry too much.
Efficiency and sensitivity are 2 different things and I'm not up to speed on the technical differences. The number you want is sensitivity, though. Efficiency would be expressed as a percentage....%age.
Personally, I think that phase data....the Inductance or Capacitance of the speaker at different frequencies is more important than a single 'sensitivity' number. This is reactance and the higher the value at any given frequency, the more power is unavailable to the load to do work. What this measures is if the voltage peak and current peak in the waveform are 'in sync'.
If you are curious about this, Google 'Power Factor' for more information. Just Skim the Wiki article for the general idea and look at the illustrations.
Tube amps are a little funny about speaker loads and when SS 'took over', a different kind of speaker came with it.
If it plays loud enough for you, in the space the system is used in and doesn't distort or run out of power, you are probably OK. You could have hi sensitivity speakers, just buying on that number, and have a worse load for your amp.
Look at the data from the link. Figure out what you can and post back::