Thanks to Mark and, was it Rob, for the information - one said one electronic systems give 70% of the improvement and one 90%.  I'd have to say I'm a bit skeptical of the 90%, but at least it seams to be more than 50% and that makes me more comfortable.  The biggest advantage of any of the electronic systems (in my opinon) is that they provide a spark advance that is a function of at least manifold pressure, giving an advantage any time the manifold pressure is much lower than maybe 25 inches.  But there is very little combustion taking place in the exhaust system regardless.  The reason the exhaust temperature rises is that more of the combustion occurs after TDC and that means less of the energy is being converted to work.  The down side of advancing the spark is that since more combustion occurs at the highest cylinder pressure, more heat is transferred to the cylinder head and piston.  The higher EGT's you see are directly related to lower engine efficiency, but in all cases probably 99.9% of the combustion occurs in the cylinder.  The Lightspeed system employs a capacitive discharge system and that will improve the ability to fire a fouled plug, but it will be less likely to be able to light an exteremely lean mixture.  The P-mag uses an inductive system and that will be better at lighting lean mixtures - but not so good at keeping a fouled plug working.

And is it a bad idea to disconnect the MAP sensor hose?  Well, like I said before, the only substantial difference will be found when operating at low altitude at part throttle and then the engine will just see less spark advance - about the same as with a mag.  Since my flying is done probably 95% at full throttle the overall "hurt" from disconnecting the hose will be small(pattern altitude is 8,000).  But I like the idea of watching the exhaust temperatures rise when I do a runup with either system shut off.  It's probably better to leave it connected, though.
Gary