You BET that they are going to be over 600Hp. Let it be advertised at 600 and come in at 580 and see what happens. If you need an example, Look at what happened to the Cobra a few years back when it's "real world" numbers didn't meet the claimed Hp. If you state a spec, you'd darn well better err on the conservative side.
As far as J.H.'s claim, knowing that he is a tuner, it would be counter-productive for him to inflate the base line numbers. If anything, I'd be more likely to believe that a tuner would post lower baseline numbers so that any mods would show an "inflated" gain, not a minimal..