Sounds high for a car with a 2.0 liter engine. I used to have 1989 Opel Vectra with 2.0 8V SOHC engine, and the consumption was around 8-9L/100km. But then again, if you drive mainly short trips in stop and go city traffic, the consumption can actually be fine.
If it's really consuming too much fuel, you should consider where the fuel goes. It can go to a fuel leak. It can turn into heat (and possibly motion, propelling the car forward). Or it can go into hydrocarbon emissions.
If the difference between normal and your fuel consumption is so high, I don't believe it can turn into heat anywhere else than in the engine. If your engine is in proper working condition, it will turn into motive power as well. If it turned into heat in the catalytic converter, it would be overheated and destroyed very quickly.
So, you should be looking at fuel leaks (very common in cars of this age, especially if the rubber fuel lines haven't been changed) and hydrocarbon emissions. For example, a bad O2 sensor can cause the engine to have a rich fuel mixture, thus making in impossible for all fuel to combust, and causing hydrocarbon emissions. Bad spark plugs also can cause poor combustion, causing hydrocarbon emissions.
I don't believe a bad fuel filter could cause increased fuel consumption, because where does the energy go? If more fuel is consumed, it has to be turned to heat, and a fuel filter won't withstand much heat. Lack of power, yes, but lack of mileage -- no! A bad air filter, on the other hand, could reduce the efficiency of the engine. In this case, the heat will be produced in the engine, and the engine can certainly withstand it.
So, in summary: you probably either have a bad engine, a bad air filter, a fuel leak or a hydrocarbon emission problem (can be caused by e.g. bad spark plugs or bad O2 sensor). Engines are quite durable, so I would be looking at other things first.
About your list of things you have done:
- Injector cleanup may give more power, but probably not more mileage (consider the "where does the energy go" argument)
- Computer and harness probably don't help mileage unless it's for some reason continuously giving rich fuel mixture, which is unlikely -- that's not the typical failure mode
- O2 sensor replacement may solve mileage issues
- Checking other sensors may be a good idea, as during cold start the car is relying on these other sensors because closed loop control of the combustion process is unavailable due to low O2 sensor temperature
