Its totally possible to hit over 40 mph on steep hills on an MTB, especially if you weigh more. I was more referring to slight grades with just pure rolling without pedaling.

Im in Austin so I frequent COTA Bike nights on Tuesdays on the track. Coming down the hill from turn 1 with other riders is a very good test of aero efficiency, because there is a slight uphill after the bend. Couple of my friends ride gravel bikes, one of them is heavier than me, and its pretty clear that that my bike is more efficient.