Maybe in the freight role it would open up the world to different types of freight... Such as delivering complete motor cars. If they could be stacked side by side on top and bottom floors, maybe 20-30 at a time?
Three questions:
1. Do you have any idea what it would take to convert a passenger airliner floor to handle roll-on/roll-off of motor cars? A LOT. To put this in perspective no airliner derivative freighter can currently do that. Just reinforcing the floor to accept pallets/containers is a huge deal.
2. Do you have an idea what it would take to install a cargo door in both the lower and upper decks of an A380? It's a HUGE deal on single deck airplane. It would be a nightmarish undertaking on a double decker.
3. Have you considered how you would lift the cars (or pallets or containers) to the upper deck of an A380? There are currently no cargo lifters that can reach that high. So A380 freight service would not only entail limiting such service to the few airports that can handle an A380, but to airports that can handle an A380 AND have special cargo handling equipment.
Airbus is not stupid. There's a good reason there is no A380F.
Also wouldn't their cruising speed be lower, for maximum economy?
Aircraft fly at max range cruise speed. That would not change because of what is inside the airplane.