To what extent can you tune a bugatti veyron in reals life?

One good indicator is the Veyron SuperSport, which makes about 200hp more if memory serves me correctly, with the same base engine. And even that is done reliably. The reliability of an engine is more impacted by the driving style, fuel quality and the tune than by how much peak power it makes. Remember, 99% of driving will be done when that car is only making 200hp, not 1000, so reliability at 200hp isn't much of an issue.