Since you pulled the fender liners, check to see that the driver side does not have a large wire loom directly above the tire. The SC has a 1" wire loom thats commonly hit/worn from the tire bottoming out. Its fairly common on cars with big rims. I mentioned the loom because you stated the sound is coming from only 1 corner.
I had a similar issue when I installed Megans on my SC. Pop the hood and double check that the strut nut is tight. Its the center nut in the middle of the coilover hat. (3 bolts around it). Mine had maybe 2mm of loose slack that sounded like you said, "about to bust through the fender". I was baffled when all balljoints checked out and erverything was tight. I laughed out loud when I realized the top nut was maybe 3 turns loose. Most coilovers use a bushing up top and when tightened, will generally hold its tension. Because the Megans use a pillowball, its much easier for that top nut to vibrate loose from the road if not originally tightened correctly.