If a BBQ Rangehood states a 2000m3/hr exhaust it means that the true rate of exhaust will between 1600-1800m3/hr. The true rate of exhaust will be even lower if you are ducting through a back wall.

Ducting through the roof given your gross exhaust is 2000m3/hr your true exhaust should be 1600-1800m3/hr

Ducting through the rear wall using a 90% elbow your true exhaust will be 1500-1620. not using an elbow your true exhaust rate will be 1000-1200