Good idea but wont work. Aside from the piping being different diameters all the way back but even the gaskets are different part numbers so there is no point in the system that is the same between the 2.4 and 2.0maybe you can swap the exhaust out with the base ilx, that might fit and work?
Adding dynamat or some sort of insulation is the way to go, only problem is that it will be a bit tricky to take apart the panels since we dont know where exactly all the clips are located. It's going to take a lot of patience.It'll be louder than stock though. he wants it to be quieter.
I think adding interior sound insulation is the way to go.