Dear Ian,
Eva Here. I've read about the 'diminishing benefits'principle many times so there must be some truth in it.
However, I did find a study once which unfortunately I didn't make a note of, which gave stats that were a bit more encouraging. They suggested that more people have diminishing returns, some get the same result, and that there is a reasonably sized subgroup that does even a bit better. I have no idea whether this study was valid.
Eva