It all depends on the spark advance schedule built into the units, and I don't know much about the p-mag.  I'm told that at high altitude cruise (low manifold pressures) the Lightspeed will advance the spark by at least 7 degrees over the base timing.  That should (will) give a substantial performance benefit at altitude.  Low-elevation takeoffs won't be any different because there is no spark advance change over a standard mag.  All I can tell is that my non-turbocharged ES will degrade significantly less than a normal plane as the altitude goes up.  It is very happy above 12,000 feet and still has good performance at 17,000.  Some say 10 to 20 percent, but I'm not sure I buy that.  "Up to 10 percent", as the ads say, is certainly believable.  If you fly a lot above 12,000 feet having a system that advances the timing is well worth it, in my opinion.  If you routinely fly at 3,000 feet throttled back to less than 20 inches manifold pressure you probably wouldn't be subscribed to the Lancair list :-)

On the other hand, I wouldn't go to the trouble for a turbocharged engine, as the engine will never - or rarely - operate at low manifold pressure for any length of time.  The only operational problem with a mag is potential cross-firing at high altitude, high boost conditions.  So for that reason you might want a distributorless system(Lightspeed, e-mag, p-mag or whatever), I don't know.

Again, the important thing is not so much about how the spark is formed, but WHEN it fires.  Regarding the question from Scott about efficiency or performance - the engine always inhales the same amount of air and fuel, so the spark advance just increases the power derived - which is the same as efficiency.  More power on the same fuel.  The difference is greater as the engine is run leaner of peak as well as at high altitude.  Both slow the burn rate, so more advance is welcome.

These comments apply to any open-loop system.  Adaptive systems, such as the PRISM, are a different story.

Gary Casey