Again i have thought of this idea before for mmy MG which has little under floor clearance but I don't really need that big an exhaust as it is basically stock. I too thought oval would be
good for the straight run under the floor as you could have a large volume of pipe that was no lower that say a 2 inch round. Trouble is you still have to much around as you say with bends
when you get to the diff unless you go stright under it like the MG does. Always wondered if the oval would upset the flow and resonant frequencies etc that they talk about with exhausts
(or whatever they call it)