As both the timing and mixture are controlled by the ECU and the engine is running OK then any severe imbalance is likely down to a sensor. Its unlikely to be timing as both cylinder sparks are triggered off the same rotation sensor.
First I'd suggest you check the exhaust manifold bolts to make sure you don't have a leak.
Then, measure the header temperature with an Infrared thermometer to find out whether its a real problem or the left just appears to be running significantly hotter. Ideally measure another M3W to see what the normal readings should be.
Second, check the spark plugs which should be about the same colour. If not either the left will be lighter so running lean or the right will be black and running rich.
As its a closed loop system the prime suspect would be the O2 lambda sensors in the header. An easy test is to swap them side to side and see if the problem transfers. If it does replace the relevant sensor..
