Just as a point of reference, I have 1 Slick mag and one Lightspeed ignition on my IO-550  Legacy, 10:1 pistons.  After 250 hours I am very happy with the setup.  Engine will start on either system, normal start is on both.   Of course it starts much quicker on the electronic ignition.  Mag check has a 20-30 rpm drop on electronic, and 60-90 on the mag.  Spark plugs wear a little quicker on the EI side, course they're only $3.00 each to replace. Only issue I've had is 1 failure early on , a hall effect sensor failed due to an installation error.  The crank position pickup board was touching the baffling in one spot, during flight the baffling moves ever so slightly which caused a mechanical interference between sensor and the rotating magnet.  This caused 2 cylinders not to fire on the EI side.  Noticed the problem during and inflight mag check, landed, had board repaired and improved by Klaus and haven't had a problem since.  I will give Klaus credit, he has made 2 improvements since then, a bigger magnet so that the space between the rotatting magnet and sensor can be increased, also a metal striker plater that protects the whole board.

Micah Froese
Lancair Legacy
250 hours