I had heard this before but 20% is too much IMO, Bob Brozman was on a guitar forum saying they were about 5-10% too fast.
Found a link
http://www.guitarseminars.com/ubb/Forum1/HTML/004431-2.html
Interesting though, the thing that makes me think is that if Johnsons recordings have been widely available since the 1960 when his contemporaries like Son House, Johnny Shines and David "Honeyboy" Edwards were very much still alive, why none of them had ever said anything if they really were 20% faster? Every interview I have ever read with them is always quizzing them about all sorts of things to do with Johnson.