I think you are going to have rasp no matter what with that setup and no cats. I have the gmmg with longtubes AND cats, and there is still a little bit of rasp. It's not bad though, and I don't mind it at all. With good cats like the new Magnaflow metallic cats comming out, there really is no reason to use an offroad pipe anyways. They flow 600 cfms!

I think this is pretty close to what a straight piece of pipe flows. Just throw a couple of these on, and that should take care of most of the rasp.