A lot of variables. It is correct that sensitivity on 4 ohm is normally measured with 2 volts and 2.83 volts on 8 ohms. These past couple of years, I see more and more speaker manufacturers labeling 4 ohm sensitivity @ 2.83 volts.
The amp is also a factor. Under normal conditions a good solid state amplifier can double its power output @ 4 ohms. With this scenario, you very well could hear a difference in a 4 ohm speaker vs 8.....(4db higher sensitivity plus 3db in power gain)
However, as mentioned earlier many tube amplifiers perform better at higher impedances and have separate taps for 4 and 8 ohms. In a good tube amplifier with reasonable power output and using 8 ohm taps, it is not likely that you would hear any appreciable difference. I believe that if you are not a head banger and have 40 watts per channel or more, you should be just fine with a tube amplifier and 86db 8 ohm speakers. I hope this helps, Tim