Here is a set of images showing the speed of combustion, greatly affected by the position of the spark plug.
It is not always good to have fast combustion duration. You will loose some low end torque at low RPM. Central spark is great for medium to high RPM. I believe carefull mapping a water injection system will extend burnt rate and improve low end torque.
Ideally, having two spark plugs firing separately based on RPM is a possible solution. You can see how fast the flame rate advanaces on the bottom image.
