So either the sums for the 'Eco boost' don't add up, or to achieve them you've got to use so little of the performance that there's little point in having 355bhp in the first place.
Not necessarily. The performance comes out of it because the turbos are able to spool up down low and give these otherwise absurdly heavy vehicles (the Flex at 4600 with AWD, the MKS likely around 4100) the power they actually deserve with otherwise decent fuel economy otherwise. If Ford would have used the Modular V8, my guess is that fuel economy would have doped to 20 MPG or below on the highway, so there are advantages to the TT V6.
Now, if Ford really wanted to do something easy and interesting, they would equip all of their cars with the "Eco Meeter" from the Fusion Hybrid. That little display shows the driver how much fuel they're using based on excessive acceleration and braking, as well as a "window" to keep speed in to optimize fuel efficiency.
*A Point on Fuel Economy Ratings*
Its a rough science, at best, and using actual EPA figures is always a bit wishy-washy. A few years back, the EPA drastically recalibrate their testing procedures to be more like the "real world" (as you requested Stotty). Most cars and trucks lost fuel economy ratings by a few ticks, some actually gained a bit as well. Nevertheless, there is always a sticker on the window that says "Actual Mileage May Vary," but overall, it is not difficult to beat the EPA figures.
Case in point?
My Celica is rated at 19/26 using the old figures. If anything, I should be getting fuel economy numbers that are worse. But, even in mostly urban driving, I've been able to pull of 28+ MPG regularly (when the weather is a bit warmer), all with very little work.
Ford's point with EcoBoost is to give drivers the power their vehicles deserve with the fuel economy that they want. So, when you're getting BHP figures that are higher than any V8 offering other than whats given in the GT500 with the fuel economy of the 3.5L V6... Its a good bonus. But, as Omnis pointed out, the sweet spot will likely be the EcoBoost equipped I4 engines... Conceptually demonstrated in 1.6L and 2.0L forms. Those engines are set to replace standard V6 options in most vehicles, or become a performance option. Everything from the Fiesta to the F150 is set to have an optional EcoBoost I4... And that, I'd say, is progress.