Study after study show the more lanes you add, the more traffic you add. Right now, based off trip numbers, the busiest two sections of Interstate are 35 between Ames and Des Moines (about 45,000 ADT) and 380 between Cedar Rapids and Iowa City (about 50,000 ADT). Outside of the DM metro, 80 is busiest between the Quad Cities and Iowa City, but it is still between 30,000 and 35,000 ADT.
Get the first two done, and then worry about 80. I like the idea of a toll road as it brings in out of state monies to help pay for it.