Charcoal always (eventually) burns at the maximum volume for the allowed airflow, regardless of the density of the charcoal.
X air will create X temp.
Btu numbers are created by charcoal weight not volume.
5 lbs softwood charcoal will burn the same time as 5 lbs hardwood charcoal and create the same BTUs.. but the softwood charcoal will appear to be much more charcoal and light much more quickly and "appear" to burn more quickly.