Opening your combustion shutter will generate more heat from any given amount of coal. As stated above, just watch you don't add TOO MUCH air so that your draft goes to -0- and flue gases start leaking out your doors.
Don't worry about losing heat up your chimney, while you flue temps will increase slightly, your barometric damper should keep your draft in check and your flue temps in check by default.
As long as you have your max burn set according to Keystoker's recommendation, you shouldn't have to worry about the amount of ash on the burn grate. You will never burn at your maximum rate 100 % of the time so there will always be varying lengths of ash on your grate, no matter how much air you are blowing through the coal bed.