Because of the differences in QB's, scheme, schedule, etc., I find it difficult to get too caught up in stats like these.
At least for RB's you can take the QB throwing out of the equation... but even then, schedule, OL, etc. come in to play.
I appreciate you doing the work theogt... and I've done plenty of research like this myself (in my spare time)... but it's just too difficult to take every variable into consideration.
Heck, you hinted at it yourself when you said that Johnson's stats don't measure up... and he's by far the best WR in the draft.
Bottom line for me is that I like Ginn's explosiveness and think he can change a game with his speed, but it will take time for him to become an effective everydown WR... if he ever does.