The corrosion is not bi-metal related. I went down this path freakin out worrying about mine and thought to replace my rivits to avoid the problem. Turns out that the rivits are aluminum as are the sills.
There are two reasons for the corrosion. For ones that have rivits, the rivits go straight through and are exposed to the exhaust. They heat up first, well before the sill since insulation is protecting the sill. The movement in the rivit and the hole allows the seal to be compromised and contaminants expose the aluminum to regular corrosion. The second and more important issue is the insulation. Whenever Vipers are driven in the water and the sill area gets wet and water is trapped between the insulation and sill think about how long it would take in that area to dry. The entire area is basically contained except for the air slots underneath so it will take days to dry. It is begging to start corroding the aluminum from the inside out.
