1997 Chevy Camaro gas milage and performance

Any ideas what P0305, P0441 and P1406 would have in common?

I ve replaced Plugs, Wires, Ignition coils, ICM Module, EGR, O2 sensor and Cat Converter. Checked Vacuum lines, no leaks seen.

The Plugs for cycl #5 appear good, except a small brownish burn mark at tip. When I changed plugs again, I used ithrium and after 100 miles, again very small brown discoloration on the 1 plug. I do not feel any real hesitation or misfire when driving.

The P0441 comes and goes. The P1406 has never left after replacing EGR 3x.

The gas milegae drop from 300 miles a tank to 200 miles a tank has been the most noticable.

Thanks