It depends on how it was advertised.
Your car will have the 296 bhp engine and not the new 313 bhp engine, hence why it has no bump. If your car was sold as a MY07 with 313 bhp then yes, yes have been conned. If it was listed as 57 plate and no reference to BHP then I don't think you have much of a case.......
Your car will have been made but not bought and probably sat at a dealers until someone bought it in Sept 07 meaning the current reg at that time was the 57 plate......