If the phase angle is small, that means the impedance is mostly resistive - which is a good thing for SETs with no feedback. But the magnitude of the (mostly resistive) impedance will still have an effect. With low resistance, the current demand is higher and the damping factor is worse. But this also depends on the transformer - the output tube sees the speaker impedance modified by the OPT ratio. I use a fairly conservative design, which means a greater tolerance for speaker impedance dips. I don't have a problem with your speakers on the 8 ohm tap - it certainly won't hurt anything.
So, while I strongly encourage you to try it both ways, I must say either is acceptable. If you use the 4ohm wiring, you will have a bit less peak power, but better bass damping. Your ears should be the test, not theory!
For more power, to my ears zero-feedback triodes are still the best. Remember that Stereomour can be bridged to make a 7 watt amp - so you could get to Kaiju-level power that way. On the other hand, a Kaiju can be bridged to make a 16 watt monoblock, so if 7-8 watts is not enough you could add a second Kaiju at a later date.
I hope that helps - sorry it's not a very clear recommendation!
