Let’s be specific here about the S550 Mustang power ratings:
3.7 liter V6 – 300 HP and 280 TQ (a bit downgraded)
2.3 liter 4-cyl turbo EcoBoost – 310 HP and 320 TQ (all new, all good)
5.0 liter “Coyote V8” – 435 HP and 400 TQ (a bit upgraded)
And here’s the full price list.
Official 2015 Mustang curb weight info.