Are there any? And are they able to beat the NSX?
Because that sounds interesting, and I haven't heard about any American MR cars yet...(Well, Saleen S7 is just one of it. Right?)
Because there aren't many MR cars in the streets.

Not a lot of cars come to mind when thinking about American sports cars that are MR. I can think of the GT40 the S7 and the Fiero. I'm sure there are more, but I can't think of them.

Well, aside from what you have already named, there was the DeTomaso Pantera, a couple of Vectors, and another Mosler, I think it was called the 900, and the GT90

Of currently built cars? Ford GT, Mosler MT900S, & Saleen S7. However, all of them could decimate the NSX and the costs certainly show that.