Guys like Ricky Williams, Cedric Benson, and Marshawn Lynch have bounced back from mediocre first teams to being great starters. Not saying it should be expected, but its within the realm of possibility. At least McFadden hasn't had a lot of mileage put on him over the last 7 years.
Cedric Benson is really the only guy from that list that could compare to McFadden.
Ricky and Marshawn had already proven they were pretty good in their first stops, even if they weren't spectacular.
Ricky had back to back 1,000 yard seasons as a Saint before moving onto Miami, and Marshawn Lynch started his career in Buffalo with back to back 1,000 yard seasons, so neither of those guys' improvement on better teams was all that surprising.
I'd say McFadden is a much longer shot to do what those guys did because he did not show the type of production those guys did. He's played 7 seasons in the NFL and only has one 1,000 yard season to show for it......and that is 4 seasons removed now.
He does have some ability and he will have room to run, so it wouldn't surprise me if he made some plays here and there, but I would not expect him to replicate anything even close to what Ricky or Marshawn did on their 2nd teams.