Cosine 45 degree=0.707
Cosine 30 degree=0.866
So at 45 degree, the power factor=0.707, meaning for every 1W required by the speaker, the amp has to deliver 1/0.707=1.414VA. Any 6 or 4 ohm speakers, combine with this kind of power factor will likely be hard to drive unless the tough phase angles and low impedance dips coincide with the less power demanding higher frequencies.
That being said, I doubt you will find too many speakers with this kind of phase angle. I have never see a graph of any speakers that shows 45 degree or higher, regardless of frequencies. That does not mean such speakers do not exist though. (Edit: Thanks to Alex, I have now seen one such speaker, the Infinity Primus 360

)