That's great - adding the ^s gives some flexibility to the function. I was worried because the form I specified (s=1) might not fit the data.

I'm dubious of the difference form. If you used it on "raw information", it could predict the same relative advantage for a 2009 WE opponent over a 2005 WE opponent, as for (say) a 1600 WE opponent over a 1000 WE opponent, because the difference in information is a small percent of the information either side has in the first case, but a large percentage in the second case.

Got the article; hope to read it soon.