Some time ago, maybe a year or two, two men that I knew of did the test for their DNA under the Johnson-Johnston Family Tree DNA project. It turned out that they were a perfect match.
These two Johnstons were from different parts of the US, one of them being New Jersey, and I think he said Pennsylvania roots. This proves only that somewhere back there in the last fourteen generations they had a mutual ancestor. The other was from Alabama-Georgia.
More Johnsons and some Johnstons took the test, but were from three to five or seven numbers off. I found that a good many of the test takers had the same first five to seven numbers in identical order. Guess this was the original group of peoples who went to make up the Scots. Ours seems to have included some Nordic inheritance.
To have any kind of conclusions to draw from this study, many more Johnstons would have to take it, then compare results.