Each individual S54 engine will be slightly different mate, some can take more boost than others. This is usually not really even dependant on the mileage it's done, but can be effected by how well it's been maintained. I would suggest that 12psi is high for an S54, especially on stock internals. I would have thought that 7-8psi is more likely. Additionally you should really always be running on 98 octane anyway, with boost this would be a MUST.