If the ecoboost vehicles didn't have to weigh close to 6500-7000 lbs they would be in the 10's. There are 12 second F150's despite the weight handicap. Imagine that in a mustang that's 2000-2500 lbs lighter. A tuned 3.5 EB mustang would FLY. A tune only Ecoboost vs a tune only 5.0 and my money is on the 6 cyl.
As for the DI being limited, well it is for now. If ford put that motor in the mustang I bet we would see a strong aftermarket for it. With that many possible customers somebody would step up and offer higher volume pumps and supporting mods. Also the diesel crowd doesn't seem to be lacking support.
The Taurus EB world is pretty large. That car comes in around 4300-4400lbs.
It's not about support, it's about the complication of it, IMO.
The new EB in the Mustang comes with an integrated manifold, that is cast into the cylinder head. That is the first obstacle. The second, is the complication of upgrading the DI fuel system. You can only make parts so cost effective - then it also comes down to the market (end user) being willing and able to spend the money on the parts, then the money to upgrade. I am sure the market will swing around for them, but it is going to cost a good bit of money to make them perform like a basic modded 5.0 model.
Time will tell. I can't wait to see what the market has in store for the S550!