Can the heat output of a furnace be increased safely by using a bigger gas valve or burner orifices and increasing the blower speed?

NO Never increase the imput over what the furnace is rated for as listed on the tag information that is on every furnace or fuel burning appliance. To increase imput beyond manufactures recommendation can and will create a safety risk and damage to the recommendation.

I answered your question by stating imput because that is the way manufactures actually rate impute. Output is nothing more than a engineers best educated estimate when the burner is running at a steady state and environment.