If it's the same engine they put in the 300M's, then my 2000 300M does just fine on 87 octane. That's all I ever use.
The horsepower, torque, 1/4 mile times they publish are probably using
89 or 91 octane. That's why they say the recommended octane is 89 - so that you don't complain to them when you don't get the 1/4 mile time that Road and Track says they got.Recommended is basically bullshit. It's when they say "required" that you have to sit up and take notice.
With electronic timing and ping sensors, and the fact that the lowest grade of gas you can find is basically 87, even engines designed to use 89 (which I'm sure the 3.5l isin't) can get by with 87.