Good suggestions from the other replies. I would like to add: Consider doing self assist when you get off the ship. This means you must manage ALL of your luggage on your own (pack accordingly) but when you get off, you do not have to take time to look for your bags. THEN, find a porter. The porter line goes through customs faster than the regular line. Then as Crusty said, cabs should be there waiting. Make sure you have all of your paperwork ready for customs so everything can go smoothly.
Last resort if you are really worried - tell guest service that you have an early flight and they may let you get off the ship just a little faster. I am not sure if they allow this or if it is even necessary, but if you are really worried, it is worth asking.
What ever you do, do not get on the carnival shuttle, you will be waiting for other people and you WILL most likely miss your flight.
Good luck! Have fun and come back and tell us all about it!
Feb 1986 Mardi Gras
Aug 2010 Inspiration
Aug 2012 Paradise
Sept 2013 Valor