[bobcowan]

Depends on the engine. The ignition system will only produce as much energy as it needs to make a spark. If it only needs, say, 50 joules, then that's all it will make.

On a high compression or FI engine, it can be difficult to produce a spark, so the added energy will make more power in that engine.

Multiple spark only works up to about 2,500 - 3,000 rpm. After that, it's too fast to make things happen in a single coil system. It gradually tapers back down to 1 spark per event. Most street engines spend the majority of their time below 3K. Multiple sparks can increase fuel economy and and decrease emissions.