Back in my younger days I was riding along with a friend and we decided it'd be fun to do a speed run. Straight road, good visibility, all that jazz. Woman on a side road either didn't see us coming, or saw we were far enough away that under normal circumstances it would be safe for her to pull out. Luckily she made a right instead of a left and the other lane was open. There was no way we could have gotten slowed down in time.
If it's a closed road or a road posted "drive as fast as you want" I have no problem with him going 150. In those cases I'd just ask "Why so slow?". Those cases don't apply to the road he was driving on, and other people with access to the road aren't expecting it. Sure, you're supposed to drive defensively, supposed to anticipate, but when someone's this far out of the norm it's not the fault of another driver - like the lady we blew by that night - if they didn't anticipate correctly. (Which is the reply I'd expect to get on here)