My first question is this:

If you're packing that much gear for a regular commute, why a backpack? Is there a specific reason that you need a backpack instead of panniers or other bike mounted bags?

I prefer to let my bicycle carry the load instead of my back carrying the load. Have you thought about using panniers or a nice seatbag/handlebar bag combo? 40 liters of stuff sitting on your back every day will get tiresome pretty quickly.