I almost let this slip through the cracks, but Tony Blengino of FanGraphs went through an exercise last week ranking each MLB team's farm system in terms of impact. Blengino defined "impact" as the number of players who "project as likely above average major league regulars," so it doesn't come as a shock that the Yankees ranked in the lower tier of this exercise.
The only true "impact" player that Blengino identifed was Gary Sanchez, although he did also tab Tyler Austin as a potential "non-impact MLB regular". As was the case with most other organizational rankings this past offseason, the Yankees' strong group of lower-level prospects that could turn into "impact" players in the next few years kept them from falling further down the rankings.
Not the most clearly defined or statistically-supported method, but an interesting read nonetheless if you're looking for a different prospect evaluation method.