Changing the heat range of a plug does not change how well the fuel is burned. The heat range of a plug is related to how well it transfers heat from the tip to the body of the plug then cylinder head. A hotter plug has a longer electrode tip so it keeps the tip hot enough to keep it clean. A colder plug has a shorter tip so it transfers heat easier to the plug body then to the head so it runs cooler in the same given motor. It is recommened as a general rule that with every 75 - 100 change in horsepower you run one step colder plug. The reason is that a higher HP motor will generate more heat and will overheat a "stock" heat range plug. The colder plug will allow the plug tip to run at the correct heat level without overheating it.
Like stated earlier too hot of a plug can cause engine damage from detonation. If you use too cold of a plug the plug tip won't get to the right heat level to keep it clean so it will foul prematurely. So it is much better to start with too cold of a plug and work your way up until the fouling is minimized.
I run a plug 4 heat ranges colder in my 340 because I am at about 550hp.
Chuck