There is nothing unrealistic about it. Look at the 1/4mile and trap speed. Look at the potential LS based motors have in the aftermarket. Its likely closer to its 495hp rating when running at the limit.
We shall see when owners get them. The Zupra is putting down far more power than rated as well.
There is literally a 0% chance the LT2 makes anywhere near the power MT says it does. The SAE J1349 ensures that each engine is within something like 1.5-2% of the engine rating. Not +30% over the factory rating LOL!