I can honestly say I once rode about three miles at 40+ mph and only putting out about 200w. And it was a largely flat three miles, with no elevation changes greater than 1 percent.
Of course, I was greatly aided by a massive tailwind brought on by an incoming haboob.
And I ended up calling my wife to come get me that day, as the wind was much too intense to get back home. When going into the wind, I was struggling to turn the 39/27. And with the wind at my side, I actually had to lean over to stay upright, which caused problems when there was a momentary lull in the wind, and I nearly fell over. Also, tumbleweed. I spent a lot of time trying to dodge them or stopping to pull them out of my rear wheel when they hit.