Adding mechanical filtration raises the capacity of the impurities that you can collect from the water in your tank. Adding bacterial filtration allows for more conversion to nitrates. Adding filtration also increases the cycle rate of the tank - how many times the water can get turned over in the tank.
However, removing the nitrates in the tank takes water changes. The BB will equalize with your bio load. If you add additional fish and convert the additional bio load with additional filtration, it will increase the amount of water you are required to change, or increase the times you do your PWC per week. For example, maybe you do 2 PWC of 35% instead of 1 PWC of 50%.
As well, to oxygenate the water you need surface movement. Although the filtration should cause additional current in the tank, you have to make sure the surface of the water has enough agitation.