if it is 425hp as placed in the other SRT8 vehicles, i see no reason why you couldnt tickle the 600hp range NA...

first, get the cats off, and get a nice set of headers...i am not familair enough with that intake manifold to know what could be done with it, but figure a bigger TB, and better cam, as well as some sort of computer timing advance, or "chip"...seeing as how in the charger/300c, the 6.1 has dyno'ed to mid 400 range at the crank, 500 should be fairly simple, unless they squeezed everything that they could out of this, which i doubt.