Fuel Consumption isn't affected by open pipe or any exhaust mods. however there are theory's behind it which backed by experience from myself and my colleagues.
1.) High Fuel Consumption due to sungaw pipe - big diameter pipe like GP Elbows with unmodified engine. This happens when your powerband or max torque shifts to higher RPM requiring you to rev it higher to attain max torque since exhaust velocity is slow affecting low RPM torque. The more you use high RPM, the more fuel it consumes.
2.) High Fuel Consumption due to waswas mode always on - you like the bassy sound of your engine, revving it a hundred times (stop light, starting it up, parking it, during cruise mode you intend to press the clutch and rev and rev it.) thus consuming more fuel.
We should have stage upgrades just like car, what the motorcycle industry is doing is like creating frankenstein. Ex: Racing Cam without chaing air filter and exhaust and etc...)
Staged Upgrade:
Stage 1:
Power Intake Filter, Power Exhaust, Sticky Wider Tires and fine tuned Carb (non FI) - all engines are built with stuffy nose. This will increase your power by 5%-20% depending on the tuning. Example is that you breath through your stuffy nose(Intake Filter) and then breath out to your mouth (Open Exhaust). You have to satisfy both... Stage 1 is more of a very popular tuning aiming almost all consumers for increased fuel efficiency with the Stock Durability. Vixion Kit is also an option since it is a stock part but bigger piston.
Stage 2: Stage 1+ High Lift Cam + Port and Polish head + Racing CDI+Stiffer Spring + Bigger brakes - A tuner's second stage upgrade.
Stage 3: Stage 1+ Stage 2 + Suspension Upgrades+ Bigger Bore kit - Racing setup starts here.
Stage 4 -infinity depending on budget.