You know I've heard this before, and believed it, but decided to check it out for myself. What I found is this:
Peters started 10 games last year, i.e. almost 2/3 of the season. According to STATS Inc. (McGahee 2005 stats), McGahee averaged 4.1 YPC on the left side, 3.6 in the center, and....ta da...4.0 YPC on the right side. Now unless he had an astronomical average in the 6 games MW started, only to be brought down once Peters took over, you might have some weight behind your argument. But seeing as how McGahee averaged no better than 5.3 YPC in his first 6 games, and even averaged 2.6 and 2.8 YPC in 2 of them, that's highly unlikely.
And keep in mind that 4 of the first 6 games were against the top-7 worst rushing defenses in the NFL. I wish I could see a season split of the left, right, and center breakdowns to get a better view, but I doubt it would tell us much more. And McNally seems to like the kid, unlike his total non-endorsement of Williams.