There is an interesting article in the most recent AutoWeek magazine, that asks if there is such a thing as too much horsepower. It's the issue with the new Range Rover on the cover, I got it Friday, but it's not on the website yet. Check their site this upcoming week it should be up soon.
They take quotes from executives at Ford, GM, Nissan, Ferari and others and ask the question how much is too much. Some of the responses are predictable, "There is no such thing as too much.."
They list the most powerfull production cars made, and the top that I recall was the Posche Carera GT with 605. So maybe 300 in the new Mustang isn't so bad.