Spadge,
Because that is what is needed to be competitive in the classes that Cossies run in (how many MORE times? ). 350bhp through the front wheels in a Vauxhall IS competitive in that class (just like the 350bhp Zetecs etc). The organisers want "ten of the best" for the spectators to watch, not "ten of the mediocre", hence why they want to see cars run a qualifying session to get in. The Scoobies ran one etc etc, the Skylines have some sort of vetting system (not sure what, but they always field 10 quick cars).

The fact of the matter is that there are limited spaces and the event is over-subscribed and the organisers don't want cars in that aren't going to be a good spectacle. This means they get to chose what is good enough.