I'm a newbie at this but I think that expanding your intake holes to 1" would increase the amount of oxygen that your coals are receiving. You can always choke them down if your fire runs too hot. Some other variables to consider are 1. the height of your coals off the bottom of the drum; I would think that 3" - 4" would allow your intakes to scoot O2 both under and to the sides of your basket; 2. the quality of your charcoal; 3. your method for building your fire.
"What are all these charges?" "Just things that we need for the house, dear."