Thanks Mark. I didn't know the duct slowed down the air to subsonic - makes sense.
I would however argue that the fan jet at the front is nothing more than an low pressure air shifter similar to a prop. It's not part of the turbine. In fact, the blades are often made out of composites as they never get hot and it's sole purpose is to give thrust by shifting big masses of air around a turbojet core.
As for the validity of the bigger/slower vs smaller/faster of airflows, I've been told that bigger/slower is more efficient for some reason. Don't really know why. But in helicopters (which I have some slight experience with) the bigger the rotor the less power it needs, that's a fact. It also gets quieter the bigger it gets, which makes sense. But a slower rotor also stalls quicker, so it's a balance (not really a concern in a fan jet design).
Most new helicopters have pretty small rotors with high rotor disc loading. This makes for fast and nimble helicopters, but they're very power hungry.
Interestingly, the abandoned propfan concept where the blades were unshrouded did reduce fuel consumption by 35% (compared to 90's turbofans, that is), but it did produce noise problems. Apparently in these fuel crisis times, engine manufacturers are taking a renewed look at the propfan concept.
Thanks for answering.