Funny you should say that
Just been out to have a look
The Silver GSA that I've had from new has zero wear on the silver painted GS alloy footpeg hangers or the rear master cylinder guard
The Black GSA that I bought 2nd hand last year, has loads of wear on the silver painted GS alloy footpeg hangers (bright shiny alloy burnishing) & the rear master cylinder guard and is like a mirror finish
Both bikes are the same age 2005 (the Black GSA is actually a month newer) and mileage is the same at c. 26000miles
So...............in my unscientific conclusion.................it is down to riding style and hard use.
I mainly ride with the balls of my feet on the pegs, rather than flat-footed
Therefore a warranty claim to BMW is unsubstantiated and it is down to rider use