Based on personal experience AKA anecdotal evidence w/o even quantitative verification, for what it's worth:

• I think the optimal point depends (significantly) on the person, the job and the work environment
• For me, 45-50 hours a week seems efficient, most of the time
• Regarding managers not clamping down on wasteful signaling: I don't think it's strong evidence, because of course managers would want the opposite to be true. For them making employees work more hours feels like the simplest way to get the project back on schedule (and the project is always behind schedule).