Actually, I beleive a doubling in perceived loudness corresponds to a 10 dB increase. A 3 dB increase in sensitivity means the speakers need 1/2 the power input to give the same perceived loudness. So, yes, more sensitive speakers are easier to drive to louder levels. Impedence correlates with how much current the amplifier must provide at a given power level. So, an amp with lower current capabilities, such as a tube amp (as a generalization) will find a higher impedence speaker easier to drive.
As you suggest, it is more complicated than that, as nominal impedence of a speaker does tell you whether there is a very low impedence at a particular frequency, or whether phase shifts at some frequencies may require more current capability. I'll leave those details to the real experts.
As you suggest, it is more complicated than that, as nominal impedence of a speaker does tell you whether there is a very low impedence at a particular frequency, or whether phase shifts at some frequencies may require more current capability. I'll leave those details to the real experts.