I guess I am a little late to the party, but I'll throw in my 2 cents anyway.
Gearbanger: The 4 cylinder ecoboost engines are single turbo. I agree, this engine won't be spectacular in the mustang, but it may have a low and mid-range torque advantage over the current v6. My wife has a '13 2.0 ecoboost escape. The boost comes on strong well below 2000rpm, making it easy to hold top gear at highway speed and its responsive around town. Compared to the 12 escape with the outgoing 3.0L v6, the new 2.0 ecoboost makes more torque where you really need it, in the low and mid range. the outgoing 3.0 was a bit high strung and had to be reved a lot more.
'12 3.0 V6: 223 ft*lb@4300, 240hp@6550
'13 2.0 ecoboost: 270 ft*lbs@3000, 240hp@5500
So comparing the ecoboost and outgoing v6: the 2.0 ecoboost makes an extra 50ft*lbs in peak torque and at 1300rpm lower. Same power rating, but 1000rpm lower for the ecoboost. Let me tell you, the difference is night and day. I have no doubt the mustang V6 3.7 vs ecoboost 2.3 will be a similar story.
The twin turbo V6 is what is exciting. Similar to what the F150 and taurus SHO have, the variable valve timing, direct injection, boost, ect. It looks like this engine can easily make 420hp in factory trim with plenty of room left to play. Ford needs good tricks for later on, you know
.
I have heard rumors the '15 mustang GT 5.0 will be direct injected (no turbo). The engine already has the bosses for direct injection cast in, so it was just a matter of time. I would guess more power and better mileage with the addition of direct injection.
I have seen a couple comments the next GT500 will have an ecoboost 5.0. That will be a nasty combination!