I tend to agree with what Dave and Ives are getting at, in theory unless you are going erxtremes (dragracing etc) chances are you will not notice/feel any improvement with the MSD. At best (and I think this is true for most buyers of these systems) you get bragging rights "Gee look what I have".

That being said I will admit that I am running a MSD 6A box on my 351 Mustang II, and they do work. When I built the engine I put on Edelbrock Performer RPM heads (they do not have a heat crossover) on 20 degree mornings the car was extremely cold boolded without the MSD. With the multiple spark at low RPM it did make it drivable until the temp came up.

If you read the fine print with the unit you will find that at higher RPM you still only get a single spark. It did cure the coldnblooded tendencies, however I did not notice any improvement above 2000 RPM.

If your iginition system is optimised already chances are you will likely not really see a difference.