Take a look at Lightspeed electronic ignition for Lycomings/Continentals.
Light Speed Engineering - Klaus Savier
They advance and retard the ignition mainly on manifold pressure but also on rpm.
They vary a std lycoming from 45 degrees advanced to 0 degrees. Magnetos are fixed at 25 degrees ( for a Std compression Lycoming).
You really notice this difference when you pull the power and rpm back in the cruise, the aircraft hardly loses any speed. This is because it advances the ignition to extract more power from the cylinder charge.
Its so good that it comes as a factory option on Lycoming experimental engines. It has been certified by EASA for a helicopter installation.
Much cheaper than a new magneto as well. I got better starting (full strength spark at low speed) and was also able to use iridium racing plugs which are a fraction of the cost of aviation plugs.