They are two separate things.
Each scientist and taxman gives you three icons for their prospective field.
An Elvis gives you two luxuries.
By doing what you said, you are changing the general populace to have more luxuries, while specifically changing individual cities to generate more income.
Say a city has 10 trade icons and one extra worker. When you have your tax rate set to 50/50 taxes and science. You will get 5 coins and 5 beakers. Make the worker an elvis and you will get 5 coins, 5 beakers and 2 luxury.
Now, change the tax rate to 40/40/20 (tax, science and luxury) and you will get 4 coins, 4 science, and 2 luxury. Make the worker a taxman and you will get 7 coins, 4 science and 2 luxury.
So, if you took the first tax settings with the elvis (5 coins, 5 beakers, and 2 luxury) and the second setting with a tax man (7 coins, 4 science, and 2 luxury) overall, you have given your city 2 more coins and only lost one science.