That issue was brought up initially with the C7 by a number of folks. When I received mine in October saw what look like rust spots (which they are) on the back face and on some of the welds. They looked like the areas you circled. The exhaust, per the Corvette Spec Sheet, is made from aluminized stainless steel, quite probably 409. 409 has only 12 chrome and no nickel compared to shinny stainless like 304 that has 18 chrome and 8 nickel. It does discolor. In addition the welds are not aluminized and will discolor faster.
Looked at my car to take some pictures of the rear muffler panel 6 months after I used Rust Reformer in the same area as you found the posted brown spot. It was fine. See picture below. However found more rust, especially on the welds. Coated these with Rust-Olium Rust Reformer. See bottom pic, before and after treatment. Make a PDF showing details of this simple procedure compared to painting these areas with high temp paint or investing in a blanking panel. See on my website: netwelding.com/reports_vette_info.htm
Just saw this thread up again so finally went and looked at mine. @3,000 miles. This was before I saw Jerry U's post. There is some discoloration. It appears to be surface and weld rust.However found more rust, especially on the welds.
15 minutes to remove lower fascia - muffler assembly painted black
Looked like more than a 15 minute job! It also notes you need a helper:
Flowmaster Instructions for Installing Their C7 Mufflers (just the part before the muffler removal and install):
1) Before you begin, please understand that the installation of this system requires significant disassembly. The entire lower rear bumper fascia must be removed. Because this is a painted body panel, you may want to consider covering it with masking tape for the disassembly/assembly process to prevent any scratches.
2) Begin by removing the rear license plate. Next, along the bottom of the bumper fascia there are several screws that secure the lower edges. Remove all of these screws (save all fasteners for reassembly).
3) On outside ends of each side of the bumper fascia there are small, plastic vertical grates that must be removed. These are held in place by small clips. Use a small screwdriver to carefully pop these out. At the bottom, where the red and black panels touch, they clip together. Use a small flat blade screwdriver to carefully separate them.
4) On each side of the bumper fascia there is a thin horizontal red reflector that must be removed. It is held in place with clips. Use a small flat blade screw driver to very carefully pop out these two reflectors. Once the reflectors are removed, you will see two hex bolts that secure the fascia to the bumper frame. Remove these two bolts along with two that are located under where the license plate was mounted.
5) In the cavity just above the exhaust tips, there are two last hex bolts that need to be removed. At this point the panel is ready to be removed. We’d recommend that you have a helper for this part. The panel is secured along the top edge with clips. Grasp the panel at one end through the vertical slot and carefully pull it out. This will take a little muscle and finesse but it should pop right off. Once it’s off, place it in a safe area out of the way.
6) This is what things should look like now. (see pic)
These comments were made by two posters indicating best to remove the whole bumper.
1st poster: Basically, pull off tail bezels bolts in rear wheel wells, Cross underneath, inside across top , disconnect a few connectors and cable for hard key lock and it pulls off. Once it pulls off, break it apart from upper fascia .
2nd Installer: When we did ours we removed only the lower fascia as I didn't want to disturb more than was necessary. In retrospect though I would agree with Glen that removing both the upper and lower sections as one unit would be much easier.