I've just read this months LAA mag and there's an interesting piece in there from the chap who had to abort his record attempt to SA. He gives electronic ignition a definite thumbs up.

I think the reason for getting especially good MPG when at low RPM is largely due to the slow burning mixture at low power settings.

For example, at FL100, wide open throttle i.e. about 21" MP, 2200rpm, peak EGT / LOP, I get the best range of about 1350nm to zero fuel. This remains more or less constant up to FL160 or so, after which it gets worse again because one needs more power i.e. 100F ROP and more revs to get more air into the engine.

Whereas a turbo engine is never short of air and thus never runs at these low power settings like 16-21" MP.

I think this explains why TB21 owners get only about 80% of my range. Of course they do get there quicker :)

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.