Symptoms:

Low power (top speed about 70mph, as soon it encounters a hill, speed drops)

High consumption (went from 8l/100km to about 12-13l/100km)

Rough idle, car often dying on it's own

 

Replaced coilpack, replaced spark plugs, tested the leads. Also it can't be a faulty injector or a fuel pump, since the car is converted to LPG and the problems are exactly the same, wether it's running on LPG or on gasoline. Did the compression test, so it can't be be valve seats/bent valves. Doesn't consume any oil.

What's there left to test/check/replace?

It could be any number of things, sensors etc. You really need to see if it has any DTC's stored, then get them reset and see which reappear as some may have been set a while ago and may no longer be relevant.

As has been mentioned on here many times if you have access to a Windows laptop the best way is to buy a modified ELM327 cable and download the free Forscan program.

It's only throwing oxygen sensor bank 1 fault, but that's been there intermittently for the last 25k miles. Could this be the culprit?

If it can't get a reliable reading from the oxygen sensor it will be forced to run open-loop, as it does during warm-up. This might well explain the high fuel consumption. If it's been running over-fuelled for very long there's a risk of damage to the cat.