The (Westin) Excelsior is close to Termini station, which is the point of origin for the Leonardo Express nonstop train service to Fiumicino Airport. AM traffic in Rome is dreadful, as I personally experienced on Wednesday morning (Dec 17, 2014). I advise the OP to consider train service in lieu of a taxi, which costs €50 and will certainly take longer to reach FCO. A one-way ticket on the Leonardo Express is €14...and it's 30 minutes to FCO airport.
Your PM status on DL will certainly make for easy check-in. There are Sky Priority check in lines as well as expedited security at T1. Also, lounge access is OP +1, since you're a STE+ member.