In Antwort auf:

I think it has something to do with the exhaust gas flow...


That's it!
X-pipe makes exhaust gas flow easier at V-engines. In this way, both exhaust pipelines are in use for each cylinder bank.
So torque and power increase while fuel consumption and noise level (inside the) car decrease.
In former times H-pipes were used for this exhaust gas compensation between the cylinder banks. But the 90° angles at the connector between the two pipelines were disadvantageous for an unobstructed gas flow.

A side effect is, that the exhaust gas can flow a little bit better, when one of the pipelines or mufflers is jammed by a burnt CAT.

Beste Grüße
Tim