IIRC, Stacey from Trucks did this to a couple of his projects. He used a kit one time, and I believe he fabbed the parts the second time. I don't rember if he reported back how well it worked. It looked cool though (was on a lowered truck each time).
I know of two 3rd Gen F-bodies that have run their tips through their ground effects. Both owners said if they were to do it again...they wouldn't. The exhaust would make everything downwind of the tip black. Once again I think it looked cool, just not practical.
IMO, if you can run it below the body...then do it. If you really need the clearance, then you could slash the pipe as it's comming towards the side. I'll see if I can dig up some pics of each of the examples.