Seems a bit to good to be true... working 20% less, and still have 'increased productivity'?

I myself work 4 days a week, which I really enjoy. However, sometimes with a major bug I make extra hours, the work has to get done! You can do more in 6 days than in 4.

For that sake, how much further can you push it, can you increase productivity 20% by working 3 days instead of 4?