You're dropping 25% voltage which will also drop current so expect about 35% less power available... if you make no other changes.
You could go to a longer and/or higher pitch prop and thus increase load and end up near the 4S power but at higher current... You'd have to check with a wattmeter to ensure yo aren't going above the motor and ESC ratings.
I have the Dynam Tiger Moth from the same series... and it does fine with the stock prop, motor and ESC on 3S 3000 mah with only marginal aerobatic improvement by using the recommended 4 cell pack (or up to 4S 4000 mah). It seems to do better with added weight vs the 4S 2200 mah recommended.
That's not the best predictor for the Waco...