While I don't think the move will happen, you happened to choose two of the worst schools in the conference. Playing devil's advocate, they'd also get access to Texas and ISU in a newly-formed sharing agreement; both of which are excellent research schools.
There are about a thousand different ways to measure the quality of schools, and they all have their flaws. But every single one would put the schools in the ACC, as a group, far, far ahead of the schools that currently constitute the Big 12. In the US News and World Report rankings, 10 of the 12 current ACC schools are in the top 100, as are Pitt and Syracuse. For the Big 12, it's 3 (Texas, Iowa State, and TCU, with Iowa State and TCU in a tie for #97).
The Center for University Measurement does another set of rankings, focused much more on research. They come up with five current or future ACC schools in their top 50: Duke (#7), UNC (#17), Pitt (#27), UVA (#32), Georgia Tech (#36). The Big 12's only top 50 school is Texas (#15).
Now the ironic thing is that Florida State isn't on any of those lists. They tie for #101 on the US News ranking with Kansas, Oklahoma, Nebraska, NC State, and others. Academically, they are a much better fit with the Big 12. But that doesn't mean their faculty or administration see themselves (or want to see themselves) that way.