Synthetic rubber factories really should produce a lot more. Their production is pitiful. As-is, the synthetic rubber factories are worse than rubber plantations on plains and forests.
For every operational level 20 rubber factory, you have to spend 1469081 lumber 1876981 cement and 74562 steel. Total production from that is a mere 200. After accounting for the number of buildings used, the production per-building is only 82.
Compare that to a level 30 rubber plantation. Cost is 661850 lumber, 661850 cement - just a fraction of the cost of the level 20 synthetic plant, and this one is 10 levels higher. Production is 432, and thats all in one building. Even doing the farming on plains gives you a production of 144.
Instead of the level 20 rubber factory and oil well you could build two level 30 plantations on plains and still have a bunch of resources left over, and get a production of 288 instead of a mere 200. Plus you save yourself the electricity cost, which would require 1/4 of an equally leveled power plant and the coal to run it.
IMO it should be doubled. A production of 400 rubber for the factory would give you a per-building production of 164.1 after accounting for the support buildings. The equal level rubber plantation on a jungle is 192 per building. That way having jungle is still very useful - your rubber production costs a hell of a lot less to build and you produce 17% more per building. But the difference between plantations and factories wouldn't be so big that you're better off using plantations even where you don't have jungle.