33% of grades awarded in IB are 6 or 7 (source: IBO stats page linked a few threads down).
About 20% of A Levels are grade A
So getting a 6 or 7 is not equivalent to an A: it is significantly easier. The worst 6s awarded are more like an average B in A levels.
EDIT: just come up with an even better way of seeing how stupid the comparison is.
According to the site above again, 66% of grades awarded are 5, 6 or 7.
According to http://www.dfes.gov.uk/rsgateway/DB/...R01-2006v1.pdf
69.8% of grades awarded are at grade A, B or C
Therefore a score of 30 with no bonus points (getting 5 in each level) seems much more similar to a candidate getting Cs in their A levels.
Working on percentages, then on average, an IB mark of about 6.3 is equivalent (just based on how many people get it) to an A at A level.
This suggests that the true comparison to a straight A student is about 38 - or what Oxford, Cambridge et al knew already.
As far as I'm concerned that article has been completely statistically trashed.