Stage 1 is an ECU flash only, on stock hardware. On an EA888 that is typically 50 to 80 hp and lb-ft for the money, which makes it the best-value mod on the car. The turbo engines respond hugely to a remap.
Use a reputable, well-proven tune: APR, Revo, Unitronic, Cobb, Integrated Engineering.
What I watch before and after: a tune raises cylinder pressure, so the health of the rest of the car matters more than it did.
- The clutch or the DSG.
- The coil packs.
- The diverter valve.
- The cooling system.
And keep the oil changes short.
For reference, what comes next: Stage 2 is Stage 1 plus hardware, usually a high-flow or decat downpipe and often an intake, with a matching map. More power and spool, but a decat affects emissions and legality. Stage 3 is a larger turbo with supporting fuel and clutch: serious money and complexity.
Stage 1 is the standout value. I only go further with the supporting mods.