Must admit I have never looked at strike rate of favourites between classes of races. If it did hold that favourites have significantly better strike rate in higher class races then my inclination would be to use this info to back AGAINST the favourite in the lower class races rather than backing with the favourite in the high class ones - they still seem to get rolled a bit too often for the short odds you get.

Maybe this would be a good basis for selection for the allup-place betting favoured by some of the posters here - I think you would still need to do some additional analysis to get a positive return though.