Hayes and Johnson have not admitted that. You're taking what's been said and extrapolating from it. Again, there is not one instance of a receiver being good before coming to the Patriots, "struggling with the playbook" in New England, and then moving on elsewhere and becoming good again. There are, however, instance of players who improved significantly upon arriving in New England. Do we just assume that was because the playbook was so easy that they had an extra jump on the opposition?
Of course not.
The bottom line is that not once has it happened because of the playbook, despite your claim that the playbook has been a barrier.
First, Donald hayes absolute admitted almost 3/4 the way through the season that he was still struggling with the playbook which led to a lot of talk around town whether he was dumber for not grasping the playbook or admitting that he couldn't to the press. I couldn't find his quote, but I did find a quote from Scott Pioli admitting he was too dumb for the Pats' system:
“We had hits and misses, even in free agency,” said former Patriots GM Scott Pioli. “Donald Hayes, I blew that one. No offense to Donald. What it is, it’s difficult sometimes to see a player in a different system. And depending on how complicated or how the challenges of your offense ... there’s the mental part. There’s some guys that can get by purely on athletic ability and skill and make a difference in college. As those players become pro players, and other people around them improve, sometimes it becomes difficult. Some of that becomes the mental part of the game.”
patriots - When it comes to wide receiver, Patriots are at a crossroads - WEEI | Christopher Price
So right there is your proof. Pioli admitted he was wrong to sign Hayes because he couldn't pick up the Pats' system and playbook.
I also found this article from 2003 on this site although it doesn't have the quote, it does say Hayes admitted to not knowing the playbook:
After a promising start against Pittsburgh, Hayes sank deep into an abyss in which he admitted he wasn't learning the Patriot playbook.
More Doesn't Always Mean Better
As for Ocho, it was more subtile. But everyone know that he was struggling with the playbook and clearly didn't know where to line up in the Super Bowl on the two minute drill.
Ok, I am out for the day.