Max power from a 4.0 Litre engine

[DailyDriver]

This might be an obvious question, but since the Falcons have a 4.0 Litre engine, shouldn’t the max torque/power to be gained from the engine be higher than it is? I mean if I change the exhaust/headers and free up the engine completely, why am I only producing 10-15kw more at the rear wheels? Is something else restricting the max power/torque of the engine?

Other car companies produce as much power (like the Lexus 2.5 Litre) or more power (like BMW) from smaller engines. From a 4.0 litre engine you should be able to get over 180kw easy, shouldn’t you??