I do not believe it will be the 6" Ultimate Captain America figure just blown up, there's sculpted detail already apparant even in that mockup photo not on the 6" version(mainly more scales around the armpits). It's based on the same buck I imagine, though. Also the headsculpt is new. Reusing bucks(ie the general body shape with the articulation already in place that the sculptor sculpts over) is not new with Toybiz, it's technically a new sculpt that requires a new mold, it seems to be done mainly for aesthetic reasons. For example, Black Panther is based on the same buck as Ultimate Cap, but the details are different. Same body shape and articulation. Similarly the Smart Hulk from Hulk Classics, and the Maestro Hulk from ML12, are based on the same buck but different molds. They're not saving much money doing it this way, the majority of the expensive involved is in cutting new steel tools for the figure.
Thus while the buck may be the same as the two up of Ultimate Captain America, they would not save enough money just upsizing the 6" mold to a 12" figure mold. I would not expect the details to be washed out like you fear. Of course, Toy Fair will tell the tale. I expect we'll see production samples of the first wave.
Here's the 6" Ultimate Captain America figure:
And the pic from the front of the thread:
Also regarding your earlier question, I do believe that Sideshow could do their own 12" line while Toybiz does stuff like this. The only thing stopping them is either one or both aren't interested, or haven't thought about it. Toybiz isn't exactly making a fortune from Marvel Studios either, and the Hasbro deal doesn't cover everything, for example DST will continue to make their 7" scale figures separately.