A gsr head will rev to 8k anyway, if your not making power up there, there's no sense in reving that high.

The GSR head will support 400whp, so you don't really have to touch it. If you want to mildly build it, put in an ITR intake cam, and some dual springs on the intake side. That will give you a little more "umph" and still be good for turbo. As well as give you a little le-way when it comes to over-revin.