How fast will 16 foot Lund aluminum boat with a 40 hp 2 stroke Johnson motor go?

depends on how much gear,gas people and prop size.

Average amount of gear, full gas, 2 people 180-200 pounds and my old 40 hp merc 2 stroke trimed out (wot) about 27 mph @ 4800 rpm using a 12 pitch prop. The new 50 hp 4 stroke trimmed out (wot)about 26 mph @ 5800 rpm using a 12 pich prop.