How much power does a racing catalytic converter deliver? And does the emissions change?

You can't say that across the board. It depends on the position, the size and the software. The number of cells, whether 100 or 200, also influences the performance.

The exhaust emission standard does not necessarily change when a racing catalytic converter is used. This is due to the catalytic converter and is also stated in the product description.

HJS Kats are available with ECE approval. You can find them here.