If a province is losing around 160 happiness per season from taxes, the -160 happiness will rollover to about +120 happiness. This will be shown directly in the tax box. Additional taxes lower the 120.
The only way to get -160 happiness for taxes is to have about 7 provinces all at 50% tax becuse of the -15 happiness to all counties for each one and the -50 for itself. Then, not only will you have an almost unlimited income, you will have a huge happiness bonus for all provinces. Not immensely useful- you need 7 procinces for it to work, but it's fun to do at the end of a game.
Also, watch out for happiness itself's rollover. Once I had a province with 100 happiness and getting 30 happiness/turn, and the next turn it went to 0- another rollover. Don't worry, when at 0, the happiness will start to go up again, but it is unsettling for a turn or two.