The streamwriter object is local to each instance of the function call. The name shouldn't matter. You should, however, put the streamwriter in a using block. That will automatically take care of the close and dispose.

As far as speed gains go, you probably won't be seeing much, and it may actually be slower. Making a process parallel typically helps when it is a cpu intensive task. In your case you are probably limited by the disk write speed. In addition, since you will be writing to multiple files, there may be disk seeks involved when it switches between the files, which would slow down the total speed. I may be wrong about the speed though, so you will probably still want to test it.