Jeremy Clarkson on Top Gear let it slip that it has 520 HP. Let's hope he's wrong as my bet is 550. I think 520 HP is the minimum which gets 100 HP per liter. No way Ford allows less than that.
http://www.autoevolution.com/news/2016-shelby-gt350-mustang-horsepower-figure-divulged-by-jeremy-clarkson-on-top-gear-92298.html
http://www.autoevolution.com/news/2016-shelby-gt350-mustang-horsepower-figure-divulged-by-jeremy-clarkson-on-top-gear-92298.html