Beyond all the speculation, the closest thing I've read to hard evidence is that ADD says their '17 pulls away from their Supercharged 6.2L Gen 1.
Here's some pro-level bench racing for you guys:
S/C'd 6.2L = 450 rwhp
Minus 500 lb = 400 rwhp
Minus 10 speed trans = 380 rwhp
"pulls away from" = 400 rwhp
I'm guessing just north of 400 rwhp for the Gen 2, and they rate it between 480-500 hp at the crank. The engine has a lot of head room, thanks to the larger turbos and 10.0 CR vs the standard 2nd Gen Ecoboost at 375hp/470 tq. Also, the new GT is going to make "over 600 hp", so 500 hp would be halfway between.