What does an aftermarket ignition do performance wise?

 

Well, most modified, carbureted engines run a tad rich. An MSD will keep the plugs cleaner than a stock ignition. Other than that, not sure if one is needed on a mildly modified engine. They are used in racing to keep the plug firing under boost, with nitrous, or with high compression ratios. In those cases, you need a more powerful spark to jump the plug gap.

Supposed benefit on a stock motor is that you can run a wider plug gap, exposing more of the spark to the A/F mixture. This, and the fact that an MSD shoots out multiple sparks below 3000 RPM, decreases the likelyhood of a misfire.

One of the rags did an aftermarket ignition test within the last year or two and found minimal HP/TQ gains. However, they noted that the engines ran smoother with a high energy ignition. This is likely from reduced misfires. I have noted this on my engines as well. I have a 6AL on my 82 CFI car.