I have a friend (a programmer) whose employer decided to try shortening the work day to 6 hours as long as the work still got done at the same pace. The owner was under the assumption that work will expand to take the time allotted. The experiment worked, and they still have 6 hour work days a couple years later.

I think the theory that work will expand to take the time allotted is true, and I bet the motivation of getting done early is a big contributor as well.

Maybe you can pitch it to your boss :)