You can do it without a tune - Are you talking about a diff back/cat back or full TBE system?

I did a full 3" TBE without a tune and it was fine - You just need a tune to get the most from your modification.

There is also an 02 sensor in the dump pipe, the common way to get around this is the resistor mod so that the ECU doesn't freak out.

+1 for Verso

Edit: Too many bourbons - Diff Back .. No tune needed.