In order to permit 100mph speeds like the autobahn, the highway would have to be engineered for such speeds. That will cost more. Also, maintenance on the highway will need to be exemplary. If you hit a pothole at 100 you are toast.
I doubt if those speeds will be permitted. Truck traffic might like the route to avoid congested cities.
In North Texas, the Dallas-Fort Worth area will be the main generator of intrastate travel, both passenger and truck. The current preferred route will run through Rockwall and eastern Collin Counties, still predominantly rural areas. A "corridor alternative" runs through eastern Parker, Wise, and western Denton Counties. It is hard to see what would motivate a traveller or a truck driver in Plano, Mesquite, or the west side of Fort Worth, relatively close to the TTC's path, not to use toll-free I-35 to drive to Waco and points south. Especially with truckers, where profit margins are thin, the toll from DFW to Austin, a multiple of the estimated $12.50 passenger vehicle toll to the state capital, would cut into profit margins. If you could drive from DFW to Austin in two hours on the TTC as opposed to three to five hours on I-35, it might be worth the expense. However, as you point out, maintenance would have to be exemplary to allow for 100 mph speeds. Design standards would have to be well above Interstate standards, which are usually set at 70-80 mph. Liberals, insurance companies, and Southwest Airlines would fight any legislation to permit autobahn style speeds on the TTC.
Additionally, while the route in Central Texas is not too far from the existing I-35, the growth in Austin and San Antonio has been to the west for several decades. For Austin and San Antonio residents to access the TTC, they would have to travel crosstown through urban traffic and pass over I-35 on their way to the TTC. Furthermore, there is no east west freeway through central Austin.
If Cintra wants to build a tollway, that is fine. However, they should not spend a dime of taxpayers' money and not rely on eminent domain.