I've owned a single engine boat with VP duo-prop, and two twin engine boats with single-prop drives (one VP, one Merc). As noted, the cost of changing 2 drives over to duo-props is very, very high. It's highly doubtful that you'd notice enough performance improvement to warrant it - and any minor improvement in fuel economy would never pay back the initial cost over the life of the boat.
As for backing, twin single prop drives and their inherent propeller torque are your friend. Practice, practice. Learn to put the helm in the straight ahead position and keep your hands off the wheel. Use the engines/drives to steer the boat into your slip. Buy a Davis Rudder Position Indicator ($25.) and mount it on the hub of the steering wheel so you can be sure it's centered before you start backing.
Port drive in forward pushes the stern to port. In reverse it pulls the stern to starboard.
Starboard drive in forward pushes the stern to starboard. In reverse it pulls the stern to port.
You're gonna love it once you get the hang of it.
** As for fuel consumption, my bet would be 18 gph at cruise (mid-20's mph, 3200-3500 rpm) +/- 10%. Overall you'll get in the neighborhood of 1.25-1.5 mpg.