The main bonuses of state property are the elimination of distance maintenance and the extra food to watermills and workshops. The 10% production bonus is relatively insignificant that late in the game.
Keep in mind that with mining inc, you will have to pay a lot of gold per turn to have it all in of your cities. State property will save you a lot of money instead. If your empire is large or far-flung, then state property is really good because it will save you even more gold per turn. The gains or having a higher research slider for the last third of the game probably outweigh the production gains of mining inc.
The food bonus to state property also rivals the production bonus of mining inc, especially if you have a lot of river cities and you like turning them into production cities. The gains to production from having more food and thus larger cities might be equal or better than the gains from mining inc. If you pick a really nice bendy river site for your ironworks city and run state property then you might even get 90 base production!