Efficient speakers v. less efficient speakers

The most obvious differences are technical and practical -- not theoretical.
In theory, all you need is twice the energy for every 3db difference. In practise however your 90db spl/1m/2,83V will need 1000W energy to reach 120db spl and will have disintegrated long before those watts reach them!
That's assuming you have 1000Watts available to you, of course.

So, higher sensitivity allows dynamic headroom with reasonable requirements -- say 50-100 watts amplification. Indeed, if you have REALLY sensitive speakers, you can get away with less than 50 watts energy -- much easier to find than 1000!

Yes, a big difference. High efficiency means lots of detail and resolution. Just try hooking up a noisy amp to a 100dB sensitive speaker and see what I mean. Inefficient speakers lose lots of musical information in the process.

I agree with both the above posts. High efficiency speakers can sound more dynamic, colorful, and detailed. The result is better impact and realism.

Ditto..these are the differences I hear between my system using 50 wpc and a friends system that uses an amplifier that puts out 800 wpc. His speakers are very inefficient (83dB) with a wicked load. It sounds a little slow by comparison on the same recordings.