A mod I have been contributing to
was released recently which introduces a new nation which has high admin castles, a cheap patrol-bonus unit, a cheap patrol-bonus commander and a cap-only mage who reduces unrest in the province by 4/turn. While I didn't set out to make that nation eminently over-taxable, things did end up evolving that way. For those folks who really want to test the viability of growth + over-taxation, MA Jomon would be a good place to try it.
I noticed elsewhere in the forum a bit of misinformation I want to disabuse people of: having a unrest reducing unit in a province does NOT reduce population as though you had been patrolling it. Again, that theory was posted elsewhere and I have personally tested it and found it wrong (test data will be provided upon request).
However, unrest reduction does behave a little differently than you might expect. Say you have a province that has an initial unrest of 50 (say from previous over-taxation or a bad event or something), is being taxed at 110% (which generates 2 unrest/turn), and contains a unit which reduces unrest by 4/turn. You'd expect to have the unrest there be reduced by 2 a turn (plus any unrest reduction you get from having friendly dominion) and you would be right. However, say that same province has 0 initial unrest, 110% tax and that same unit stationed there. You'd expect to have 0 unrest each turn (since it's being increased by 2 and decreased by 4) but instead you'll have 2 unrest at the start of each turn. That's because the unrest reduction of your unit happens before the unrest increase of your over-taxation. In fact, it seems to work like this:
1) lower unrest from abilities
2) collect income
3) raise unrest from taxation
?) I'm not sure where in all this lowering unrest from patrolling goes...
So, say you have a province with 100 of these unrest reducing units and you are running 200% taxation. What that means is that you will be getting money from that province at 200% each turn (as though you had zero unrest) but the province will have 20 unrest. That will reduce the number of resources that province has to recruit units but it will NOT reduce the income that province is generating. In that situation you are effectively trading some of your recruitment capacity for extra money.
This presents a new taxation scenario for you number-crunchy folks to chew on. Oh and
try the new mod out; I think you'll find it interesting.