In case anyone cares I'll mention I just got a pair of Pirelli P-Zero Race in 40-622. They measure about 40.5 mm wide on 25 mm rims and are almost completely slick. My first impression is they're fast, supple, and comfortable at 3 bar (44 psi). On poor to good pavement they feel and handle like big road tires. It's been raining here so I haven't taken them off road yet.
FWIW BRR rated them as gravel tires and ranked them #22 out of 117 on rolling resistance, #7 on wet grip, and #5 on puncture resistance, by far the best combination for any tested tire 40 mm or larger.
