One thing to keep in mind is that the Republicans intended originally to use a combination of restricted franchise for ex-Confederates, legal voting for former slaves and a newly created network of patronage in order to create a new, Republican-supporting South. The terms "Carpet-baggers" and Scalawags are obviously pejorative, but they also represent a real fear on the part of the Redeemers that the Republicans might succeed (and thus doom the Redeemers plans of a restored white hegemony).
Now OTL that all fell apart with the collapse of Reconstruction, but the Republicans ITTL might not realize that. If they are pressing for a more radical reconstruction as suggested by the OP, they probably anticipate doing a better job, and might continue attempting to conduct Reconstruction rather than acquiescing to the Redemption of the South. As such, changes in borders might be more minor than people are suggesting (and hiving off solidly Republican areas from more divided states might be seen as counterproductive; better to have 11 states you'll always win 55-45 than to have 2 you win by 80% and 10 where you're 50-50).
In the aftermath of the Civil War, a Radical Republican-controlled government might be inclined to think big and overestimate their own capabilities even more than OTL. Why settle for creating a few new Republican states when you can turn the entire South into a solid GOP stronghold? It won't work (anymore than OTL) but they may not realize that.