Im not sure on the specifics yet but we do know for a fact that taxes are not calculated just by the base population before specializations. You can test this by moving a spec away from your city, and you will see an immediate drop in population and if you check city hall you'll notice your tax revenue has dropped as well. Theres another thread you guys should check out in discussion forum called "Wealth forecast- luxurious" for ongoing discussion. It appears tax revenue is based on a combination of base population + pop boost from specs. (Maximum boost of 400%).
One question we're still working on is whether land value/wealth forecast and building grade has a direct influence on the amount of tax revenue we collect or whether its just a prime indicator of high revenue. Currently I'm working on an experiment that will settle this. It will be a couple days before I have results as I'm currently working to isolate the test.