It should be much easier to use the editor API than SendKeys. For example, here is a C# console app that lauches ACE home page and change the text in the current demo editor:

class Program
{
    static void Main(string[] args)
    {
        ChromeDriver driver = new ChromeDriver();
        driver.Navigate().GoToUrl("http://ace.c9.io/");
        driver.ExecuteScript("editor.setValue('the new text here');");
    }
}

editor is the variable in the page that corresponds to the ACE editor instance, and I've just used one of the first API samples detailed here : Working with ACE