I was always under the impression that the soot on the bumper was caused by a rich condition, not necessarily oil burning - perhaps someone else can comment here?

I'm also not sure I'd be comfortable running a heavier oil in an engine such as the C5, though I know some do. You might want to first try switching to a conventional oil, as some people have reported that this aleviates much of their burning problems (it's also been my observation that people report more burning problems with Mobil 1, even compared with other synthetics). You never mentioned if you are actually experiencing oil loss between changes. Is your level normally low at oil change time or when you periodically check it?