Lean condition??

[C5GTO]

I just welded in new sidepipe mufflers (longer version of story here: Question about rattle in CR sidepipe ) where the original mufflers were CR1 (very, very retrictive) and the new mufflers are Flowmaster Slimline sidepipe. I took the car for a test drive and the change in performance was phenomenal with the mufflers being the only change.

I took the motor up to 6,500 RPMs for gear changes 3 times over the 15 minute test drive. Upon returning home, the sidepipes looked like this:







The mild steel portions turned blue (before and after muffler), ceramic coated (collector and turn out) and stainless steel (new muffler) portions had no appearance changes.

Is the blueing a sure sign of a lean condition or is it "expected" that the proper A/F ratio would turn mild steel blue under this sort of test drive? The motor is a 5L Ford with EEC IV EFI, mass air meter, and injectors sized for up to 450HP. The motor and ECM were dyno tuned a few years ago (producing ~ 300 HP) and "chipped" to provide proper A/F. I have made no changes to motor or ECM until this muffler change. I'm worried that these new mufflers flow so much better that I've now introduced a lean condition.