Engine loss of performance and MPG after battery change. CEL is on and gives DTC P0103.?

Not 100 percent sure but i think 0103 is maf ckt range performance. It would be the code that would set when it was not plugged in. A dirty maf should not set this code. your three likely culprits are harness, ecm, or maf. I think that I would change maf first. One way to check some is to unplug the maf and see if it runs better. that would put it in limp mode.