Extra Charcoal would make the mixture slower, perhaps adding more KNO3 would speed up the reaction.
In my experience with KNO3/Sucrose as propellant, adding a small percentage of charcoal noticably increases the burn rate of a 65/35 mix. The added charcoal seems to reduce the amount of caramelisation that occurs. Mabey a very small percentage of added charcoal will do the same for a typical 50/50 mix? Of course, you could just use a propellant ratio to make a lot of smoke quickly, but this will cost more as more KNO3 is required, and it will be a little more hazardous than a 50/50 mix.