One problem with time boxing is that it isn't the actual terminal goal - when trying to be self-motivate, there would be an extra level of abstraction between effort and reward, as you'd now be self-motivating to timebox, rather than to do anything inherently useful.

Besides, it fails the "too good to be true" heuristic. If it actually persistently improved productivity in a meaningful manner, you'd expect it to have been better adopted widely by now.