Oh boy I sure love getting opinions that diverge from the same starting point and go 180 degrees!  Perhaps they who are knowledgeable with no axes to grind would fill me in with details.  The situation involves comparing/contrasting a TSIO550 with dual mags vs dual electronic ignition.  The route that I chose was dual LSE, hoping that the variable advance would give me higher power at cruise and better fuel efficiency, and would eliminate mag failure with age or high altitude. Today I spoke with an engine rebuilder who stated in his dyno testing that an IO540 (factory rated at 315 hp and rebuilt for 340 hp) yielded only 280 hp when run at 2700 rpm WOT.  They attributed that power loss to the spark advance profile built into the LSE and so always recommend dual Bendix mags to their customers.  I would assume that their test numbers are valid for the specific engine tested that day, but in general should users of dual LSE ignition expect to sacrifice such power?  Have any builders swapped out and tested both configurations?  Theoretical or theological arguments accepted. Robert M. Simon,  ES-P N301ES