Re: running track pipes causes lean mixture & damage?
The ECU can't see exhaust pressure. It either uses a signal from the oxygen sensor when running closed loop under moderate load or the air flow meter in open loop conditions (i.e. cold start or WOT). Reduced backpressure can improve scavenging, but the system should compensate. With carburators altering the exhaust affects scavenging and you could get into a lean condition, but fuel injection got rid of that problem.
I have heard that the AFM only works up to some nominal speed beyond which it runs off of a map, but I can't confirm that. If it was the case you could get into a lean condition.
Looking at the issue another way, if what he said was true all six cylinders would show lean.