A hotter spark will not burn more fuel. Upgrading a coil on a stock ignition, for example, will do absolutly nothing. You only need to increase voltage when you increase the cylinder pressure... because of the added resistance not the added fuel. Big Jim is right, a spark is a spark. Its job is to ignite the air/fuel and start the combustion process.
First off, the electrons jump from the electrode to the ground. Air is NOT a good conductor. In order for the electrons to flow from the electrode to the ground, they must overcome the resistance of the air. What happens when electrons encounter resistance? Heat is created. What is a spark? A heat source. You see, its not the voltage that ignites the fuel, its the heat created when electricity is subjected to resistance. If the electricity caused the combustion process, we'd all have plugs that ran a small wire from the electrode to the ground.
So, what happens in the cylinder? Well, that piston has just pushed all the exhaust gasses out of its cylinder and is currently on its way back down. The intake valve opens, and the vacuum created draws air/fuel mixture into the cylinder, much like a syringe pulling up blood from your arm. The intake valve closes as the piston begins its compression stroke. The piston has now travelled a majority of the way up, squeezing that mixture tight. now, the piston is just a hair before TDC, and the spark plug ignites. Lets pause this picture, and take a closer look at what is happening at the electrode.
Electrons have just passed through the air, creating heat. Any fuel particle in the imidiate area of the heat source combusts. As the fuel particles combust, they use the oxygen in the air around them to burn. This creates a chain reaction. The unburned fuel particles next to the ignited fuel particles begin to combust, and the process becomes exponentially faster. The flame travels from the electrode to the piston. We call this the "flame front". You see, the "explosion" is actually a flame traveling very fast from the original heat source to the piston... not all the fuel particles combust the same time.
Back to inside the cylinder. We left off where the piston was just a hair before TDC. The spark plug has just fired and the spark has ignited the fuel particles in its imidiate area. We now know that the flame is on its way toward the piston. Well, the piston has already reached TDC and is beginning its journey downward again. The flamefront now hits the piston with impact, driving the piston down with force. This process is timed perfectly... at idle.
But what happens when the motor revs? Well, the piston speed increases but the rate at which the flame travels remains the same. Well, this isn't going to do much for performance, is it. Because when the sparkplug ignites the fuel (just before TDC) The piston is traveling so fast that by the time the flamefront reaches the piston, it has already been traveling downward. The flamefront will push the piston downward, but not through the whole stroke. Hmmm, how to get the flamefront to begin pushing on the piston sooner? (like it was at idle) Well, we have to advance the ignition. Now what happens?
Well, the piston is on its way up, again. But this time, the spark plug fires a lot sooner because we advanced the ignition. The flamefront has started its way toward the piston, but the piston is still moving up. The piston, however, is moving so fast now that it reaches TDC and is beginning its down stroke. At this time, the flamefront drives the piston through the entire stroke, instead of "most" of the stroke.
Scenario 3. High compression or boost...
Now, the piston is on its way up again. This time, there's a hell of a lot more air/fuel in the cylinder. Its being squeezed very hard. This increased compression squeezing more air and fuel together is an excellent recipe for releasing energy. The piston nears TDC once more, and the spark plug fires. This time, the flamefront travels much faster consuming air and fuel particles as it heads for the piston. Well, the piston isn't ready for the flamefront yet. It's still at TDC. But, the flamefront doesn't care, it just slams that piston. But, the piston can't push the crankshaft if its at TDC, can it? All that energy does not turn the crankshaft, but instead bends or breaks the internal parts.
This post is very long, and I'm sorry for that. But by taking you inside the cylinder and showing you what happens, its easier to understand how it works. The spark only has to be hot enough to start the flamefront, any hotter and its not really doing anything. The only reason to increase the voltage is under higher cylinder pressures, so that the spark can be made, not so that it can be made hotter.