Since speculation seems to be the issue here, I'm not sure that testing would be of any interest. The only reason to "tune" one of these cars for a performance problem would be to observe the plugs from an analytical sense.
The word "tune" as used in these forums always gives me a chuckle. That engine management system tunes every variable at least twice a second. It does timing so well that it varies each cylinder each stroke to a total of fifteen degrees variation cyl to cyl. If it don't work right its broke. It can't be "tuned" ROFLMAO. IT CAN BE TESTED though.
The first test I would do would be to monitor the O2 sensor both for closed loop operation and switching times. I would also check for codes on the self diagnostics and check the adaptation factor set in the diagnostics. I think the cars adaptation is limited to 20%. It gives diagnostic insite to view the additive and multiplicative addaption values in the controller. The engine and the fuel system change as they age and the control system recenters itself (adaptive value). By knowing the overall value (rich or lean and the amount)a direction for diagnostics is gained.
Actual values of all sensors are read using a scanner. These often need to be confirmed by scope or other means to verify the controllers interpretation.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician