I wish I could find the study I read last year that found only about 20% of hours spent at work by a typical worker are productive, but I'll have to settle for this: http://www.nytimes.com/2007/05/31/fashion/31work.html
Working fewer hours does not mean you do less work. Indeed, the improvement in efficiency can mean you ultimately produce more. It's like if you drive a car at 100mph everywhere, you'll use more gas per mile than driving at 55mph (indeed, this was why that limit was set).
(Update: Heh, just noticed someone else used an automotive metaphor above!)