The Windsor the Hour is steel.
Why wouldn't you do what basically everyone else does and get an aluminum bike with a carbon fork? Is there really that much of an advantage to having the rear stays also be carbon? Is there some application for this fixed-gear road racer that you have in mind? I would think that the Windsor would be exactly what you are looking for in terms of comfort for long distances. There really isn't going to be a good compromise bike that lets you actually take a fixed gear on road races and win sprints. That's just not what fixed-gears do, and if you aren't doing that then why the need for stiffness?