What bike were you on that the steering grew light? I've gone 58 mph down Tram Way in Palm Springs Calif on a steel bike and the bike never felt light. Racing pros have exceeded 75 mph many times on all sorts of materials over the 100 plus years that race has been ran, and I never heard anyone complain the steering went light. I took my Lynskey to Pittsburgh and found myself doing 48 going down one of their many hills and never felt the bike going light.Because there are old steel bikes doesn't mean that carbon fiber isn't longer lived properly made.
I absolutely agree that there are very few places in which you can flex any bike enough to be disturbing but there are several places around here. And it isn't a welcome thing to have the bike doing 40+ mph and have the steering grow light.
So you have to explain that further.