I've got a current Quarq and it's the equivalent of a P2M in my experience. Pretty much fit and forget, and power readings seem accurate and consistent.
Is yours the Saturn? I have zero knowledge about their models. Sounds promising though - worth a stab anyway. The Shimano unit crossed my mind too but it's spendy new.
Anyone used a Quarq Saturn? How does it compare with P2M, SRM, etc? What quirks does it have?