OP, first of all I am no expert on these matters and I am just working my way through the education system maze. From what I observed, the A/A* results suggested by G55 show a good variation between high end grammars and Comprehensives. The gap, however, becomes narrower or even reverse when it goes down to low end grammars and high end comprehensives.
This is a link for 2013 GCSE results - http://public.tableausoftware.com/profile/#!/vizhome/EnglishSchoolGCSEResults2013/GCSEResults2013
(I don't have a URL handy to check for 2014. Someone may have it)
I used this link to see the results of many comps and grammars in different areas as we had a relocation coming up. I was surprised by some of the comp results, e.g., Waldegrave school in Twickenham has almost 50% girls receiving A/A*. There are some other comprehensives with 30 - to 40% achieving A/A*.