I'd spend your money on a new bike. At 21.5 lbs, you probably aren't going to get significantly lighter without going to a complete Ultegra or DuraAce groups and better wheels. Then you'll be spending more than what you would on a decent tier model of the three bikes you listed. And you probably should add a Cannondale SuperSix EVO to your list.
If you can realize and get out of your mind that race fit bikes aren't endurance bikes then you'll be way ahead of those that think that way. My race fit Tarmac is much lighter than the Roubaix and is every bit as comfortable on long rides. It's lighter weight has made local riding more fun for me and there are now fewer hills that I even have to worry about as the less weight saves me energy so I'm able to climb the few remaining tough hills and have more energy left even at the end of a Century ride.