I believe that Bucks schools get the highest results in the country (along with Surrey) and that this is because our upper schools get very good results as well the grammars. Schools like Waddesdon get results that are better than good comprehensives. Just because we have grammar schools doesn't mean the uppers aren't very good in fact it means they can concentrate on ensuring their pupils get the best grades they can allow the pupils at the top of their sets to get excellent marks and still being able to offer a wide range of courses to suit the ability range - not having to deal with the 30% who go to grammar allows them to focus on getting the best for the rest. Has someone got access to the data that shows upper schools in Bucks do better than comprehensives in neighbouring boroughs? I believe it is available.
Performance data for each school is on the DfES site. It's not straightforward to use it to compare LEAs, because independent schools are included in the summary figures.
You say "schools like Waddesdon", but surely it is unique: a CofE school that selects 15% of its intake based on music grades, and is 5 miles from the nearest grammar. It may be that Bucks uppers do well compared with comps in London boroughs, but that's not comparing like with like. Comprehensives in Herts (excluding partial selectives and schools with an unrestricted intake near them) score significantly higher than Bucks uppers in the GCSE tables.