Well, it's illegal.... so I assume you're talking about doing it yourself, because your tuner / mechanic shouldn't be doing it for you.

Stage 1 is designed to work with the OE DP / cat. Stage 2 on these is just a tweak to that file for people who run a bigger DP so it doesn't start throwing codes from confused air sensors. As in, Stage 1 + Test Pipes File = Stage 2.

The DP is adding any extra HP, the tune effectively doesn't change.

If you run decatted with stage 1, your engine could start throwing codes because the lack of restriction is unexpected.

Invest in a good DP with a sports cat, or just enjoy stage 1 as it is. Or come at it from the front and upgrade your intake; you'll hear more induction sound from doing that.