4legs,

I recall a few years ago someone writing somewhere that if all Betfair faves were layed (as he/she had done) then a loss would be incurred. I can't remember whether the period of time was mentioned. And this was before Betfair S.P. - so the person may have layed them had a higher price that what they started. However, it would be expected this would apply to some faves and the opposite to others.

So I think your theory might have legs (pardon the pun).

I'm sure many of us would be interested in your findings.