Matt,
I agree with you that a first move to a Cut Off Date of not later than about 01AUG07 is quite likely for several reasons.
That would only leave 2 months for any later new I-485 to be approved before dates likely retrogress in October 2013, since they could not be submitted until August at the earliest.
I would say that (bordering on) zero are likely to be approved within 60 days of submitting an I-485, perhaps rising to 55% tops if they had 90 days to be adjudicated. That's assuming dates only move beyond July 2007 in August/July and for those received on the first day the VB becomes effective. Those figures are based on approval times last year for the period when new I-485 had to be submitted.
For EB2-ROW cases this FY the respective % approval times within 60 and 90 days are 11% & 27% respectively (on a much smaller sample size).
Also, something that may be a factor and unlike previous years, a large number of EB3-ROW-M-C cases are going to be ready to adjudicate in Q4. Their dates need to move well beyond July 2007 in the May VB at the latest, which will put most approvals in Q4 and in competition for adjudication resources.