"would the decreased efficiency be a concern? "
Short answer: "probably not". In fact, the possibility exists that the new speakers will play louder.
Not knowing what the new (or existing) speakers are, I’m going to render a guess and predict the new speakers are larger -- i.e. bigger, and badder.
This being the case(?):
The existing speakers, by comparison, will have smaller and/or fewer drivers. All speakers have limited dynamic headroom via finite speaker piston area, travel, etc. Therefore, even though nothings sounds "wrong" with your current speakers, there is audible (and, measurable) dynamic compression which will improve when more and/or larger drivers are introduced into the equation (and, the listening room). Even IF the sensitivity of the new speakers is (somewhat) less, this could be more than made up for in gains in dynamic headroom because of more/larger drivers coupling with the air in the room.
As others have suggested, pairing your exising rig (a lot of audiophiles like to refer to their stuff as a "rig") with the target speakers may be most "efficient" way to arrive at a conclusion.