Depends on the engine. Some engines respond well to having the timing more advanced than the original magneto. It is much easier to optimize the timing using EI, and if the engine likes a timing different than the mag had, you will see an increase in power. I don't use magnetos on any of my many flying gas conversions.

My brother has a plane with a Ryobi on EI. The plane needed a good bit of weight added in front to balance. I said "Well, we can always put the magneto back on it for weight." He said "NO WAY, Real model airplane engines don't have ugly magnetos!!"

AV8TOR