[MENTION=73434]Rigliner[/MENTION] removed his lower grill and said it made a noticeable difference (he said probably 25% more output). On the 06+ the lower grill unbolts and comes right off, but on the earlier models there's no way to do this from what I can tell. I'm satisfied with the output, and the whole point of this was to be bolt on with no cutting/drilling into the bumper, so I haven't cut out the lower grill. I have considered cutting it out and trying to rig up a satoshi style replacement, but again, the output is plenty for me and it defeats the purpose of this install.
I started out with mine on the roof but it made way to much noise. That along with being very shaky is the reason I moved it. This was the only thing I could come up with that was completely bolt on.
There was another member who mounted a bar to the factory rails, it was a single row ~40" bar. Turned out very clean and if I didn't already have this setup I'd be considering doing the same.