A magneto is often a comprise, its not the best for full power or a smooth idle. and if its set up for max power then often the idle is very bad. There is also the drag from the magnets.
With a battery system you can build a mechincal system linked to the carb to change the timing so its right for idle ar WOT. The CH Syncro Sprak is a computer timing control that does it all for you with no moving parts.