Probably the fan is direct on the motor shaft and in order to move more air will require more horsepower, which will require an upgrade in electrical components to carry more amperage. Most of the systems I have encountered have low register velocities. You may want to balance the air flows per room. You may be dumping a majority of your air at the closest register to the air handler, which might need to be restricted to push the remaining air further downstream.
Another option may be to add an auxiliary duct fan in the duct work but that would not be my recommendation, this would lead to a higher velocity across the filter (with any fan capacity increase) which is not desirable.