I'm no engine expert, but my understanding is this:

A more powerful coil will allow you to produce a bigger/hotter spark, which will allow you to burn more fuel, making more power. You will also probably want thicker wires and to re-gap your plugs slightly larger. Its not necessarily worth it though unless you are putting more fuel into the cylinders, like from a weber carburetor, oversized valves, etc. Even then, its not gonna give you a huge amount of horsies..

-Scerb

http://www.scerbfab.com/carb_faq.html