While we don't have official information from Ford as to the final specs of the 3.5-litre twin-turbo EcoBoost engine that will power the 2017 Ford Raptor, we can put together a pretty good guess as to what to expect.
The 3.5-litre twin-turbo EcoBoost engine already sits in the "regular" F-150. There it produces 365 hp and 420 lb-ft of torque. We also know that the previous generation of Raptor boasted 434 lb-ft of torque and 411 hp.
The 2017 EcoBoost Raptor will have to surpass the figures of both of those vehicles. The current wisdom is that a horsepower of approximately 450 is likely, and torque will be at or above 440 lb-ft.
It is entirely possible that Forza made an educated guess on those numbers, but it is also possible that Ford did give them numbers. I think the fact that the 2017 Ford Raptor is featured on the case for Forza 6 makes it more likely that the numbers are indeed accurate. Evidently, Ford has already decided to use the game as a way to prominently market the Raptor. That means they have a relationship with the game developers and also would likely want the game to accurately represent the vehicle.
I'm a bit skeptical to trust the stats from a game but since Ford is a partner of Forza, it could be possible.
Forza is one of those games that's closely connected to the automotive industry so although at times they may not show real numbers, when you really look at it, they come close enough, enough to give you a sense of what to expect in production.