The RC one won't fit properly due to the contours of the bumpers being different.
If you look carefully at the cross section of the rear bumper on the RC, the lip is horizontal at first and curves straight away. On the IS, the cross section is horizontal, then theres a slight decline and then it becomes vertical.
Excuse the crude picture of worktops but it illustrates the bumpers well. The RC has a single pencil round. On the other hand, the IS has a single bevel edge.
Additionally, I don't think the RC has a stainless steel plate as an option. Its the same brushed alloy look that is available for the IS from RGM. The stainless steel plates are labelled as such and usually have sipes in them.