I bought some on clearance, Heavy Metal Switch Grass. It is about 3ft tall in the pot and very root bound. It is nearly 100 today. I dumped them out of the pots and used a bread knife to cut them in two because they were so pot bound. Then I dug a hole, and back filled with some looser soil and watered it well. My question is, should I cut the tops down? If so, how far? I know this heat is not the best time to replant, but they were doing so poorly in the pots I figured I had nothing to lose.

If the tops are showing any green I'd leave them be. If you do cut them, you might want to leave those sheaves over the top of the rest of the plant for some shade while they're struggling to settle-in. If there's not much left, a lawn chair or something similar would work. It's pretty hard to kill the grasses, so if they have water they'll probably be okay anyway.

They are fairly close to the house so keeping them watered will be easy for me. I hadn't thought about shading them though, will put something together.
Thanks!

I would definitely provide a little shade. Our sun gets so intense and conditions are so dry here in July that it is definitely necessary. It may take awhile for the grass to look "good" but as chelle said, they are pretty tough.