"County" is such an arbitrary designation by which to look at data like this, precisely because of the outliers. If you are going to compare by arbitrary geofenced areas, why not go more granular and compare by zip code?[1]
Example outlier: Brooklyn (Kings County, NY) is a single county, but if it were a separate city would be the 3rd largest city in the US. The tax statistics for this gigantic county (by population, not land size) can vary each mile you drive down the road.
The entire hypothetical premise is based upon NYC breaking into 5 cities (one per borough), therefore making Brooklyn 3rd largest after LA and Chicago.
Example outlier: Brooklyn (Kings County, NY) is a single county, but if it were a separate city would be the 3rd largest city in the US. The tax statistics for this gigantic county (by population, not land size) can vary each mile you drive down the road.
[1]: IRS 2008 .csv tax data by zip code (large) http://www.irs.gov/pub/irs-soi/08zpall.csv
[1]: Docs and other formats: http://www.irs.gov/uac/SOI-Tax-Stats-Individual-Income-Tax-S...