If you focus fire you can go way above 10. In my recent game the spaceport city had 13 Industrial Zones within 6 tiles, 1 more hit it from 9 tiles away boosted by Great Engineer and in addition I must have miscalculated one as it turned out to be 7 tiles away when I could have placed it within range as well. In general you only need one super production city, the rest will be hit by slightly fewer IZs, but still enough for them to build anything but space parts very fast.
Industrial Zones pay back way faster than anything else in the game. With a tight empire, even a moderately placed Industrial Zone should hit 5 additional cities with AOE buildings. Getting all of them up costs 175 (workshop) + 355 (factory) + 525 (powerplant) + whatever the Industrial Zone itself costs. Let's say it's placed in the mid game and costs 245 production for a total production cost of 1300 to get everything up. Assuming weak adjacency bonus of +1, the yields/turn are 1+2+6*3+6*4 = 45. The hammers invested are payed back in less than 30 turns. Hit 2 more cities and have a bit more adjacency bonus, payback time goes down to 21 turns. This doesn't take into account the additional GPP which might also be immensely valuable.
Let's compare to something else, like commercial hubs. Assume the commercial hub is placed instead of the Industrial zone for 245 production. Your trader gives +5 production. This takes 49 turns to pay back. Food yields and additional yields might change the equation to make it pay back faster, however, once the Industrial Zone has payed back the initial investment, it will very fast outperform the commercial hub at 45 production/turn. Don't even get me started on commercial hub buildings. For a direct comparison, we should look at the cost of buying them compared to their yields. Payback times for markets, banks and stock exchanges is 140, 212 and 203 turns respectively. Those buildings are absolutely horrible. The only thing that could make them even remotely worth considering (other than 2 banks for eureka) is the late game culture boosting Great Merchants in a cultural game.
I do agree that science is important. I usually place my campuses as early as possible. Calculating a value for science vs. production is quite hard. I've found the banks come at a time when I can spare production for a couple of markets and banks to save >700 science on banking and economics, in some other cases it's a more difficult equation.
I haven't looked into faith buying. What is the faith cost per production to buy units? If you build a holy site and all the buildings, how long does it take for that holy site to earn you the faith to buy the same production worth of units?
Everything above applies mainly to science victories, possibly to cultural victories as well. A domination victory doesn't need much of an economy at all.
