No cat in the up-pipe on a 2003> only 2001-2002.
The decat downpipe for the newage should have the lambda boss in the bottom for the lambda to fit into. After a while the CEL will light with P0420 CAT efficeincey below threshold
This can be bypassed with a resistor fix, or reflashed with the CEL disabled (5 min job to reflash standard MAP with CEL disabled) or would be done as part of a remap.
Without a remap you may see slight boost spiking and a slightly leaner mixture. But they run as rich as hell, so not really a problem.