At 40+mph, aero is playing a big part, still pedaling, without being able to add power, you'd need more gear as mentioned, will actually hinder your effort. Aero and weight will def matter, I've had rides where I could sit full upright and still pass lighter riders on descents.

There was a really short -15% hill on my old regular route, I used to sprint on the flat leading to the descent and then tuck down (just a regular tuck, no super tuck for me lol), I could easily hit 50mph on that short hill, but I'm 185lbs and that was with a somewhat aero setup (tight kit, aero road helmet, deep wheels).