After Tuning, will this make a difference?

[steve1178]

When I get my car back from the tuning place im going to get a De-Cat pipe (Test Pipe), will this make a difference to the tuning. Could it make it run richer, leaner..?

How much power would the De-cat pipe make with my setup!

Here it is.

Civic B16a Hatch 1995
9:1 Forged Pistons
Eagle Rods
440cc injectors
Revhard Stage 2
Dastek UniChip (Timing and fuel Control)

and a few other bits and peices!

Thanks for the help guys!