Just read this analysis on the relative 'difficulty' of all relevant hockey leagues against each other. While certainly not unequivocal proof, it seems to be pretty well thought out and uses reasonable statistics to make comparisons. Anyways, found this pretty interesting. Here is the thesis:
Summary
The league difficulty for the largest NHL feeder leagues is shown below:
League Difficulty
NHL
1.00
Russian Elite League
0.83
Swedish Elite League
0.78
Czech Republic League
0.74
Finland SM-Liiga
0.54
Deutsche Eishockey League
0.52
AHL
0.443
IHL
0.43
Switzerland National League
0.44
NCAA
0.41
Canadian Major Junior
0.29*
* The difficulty of Canadian Major Junior is lower than predicted by this method since major junior players are 20 years old or younger and consequently experience significant skills growth from year-to-year.
http://www.behindthenet.ca/projecting_to_nhl.php