Ignition timing is computor determined off the crankshaft on the 103 motor. The timing chain wouldn't be involved.
What kind of plugs are you using. I have seen platinum plugs cause some weird performance problems. Plug gap too close can cause idle problems that fuel could affect.
As to the general performance issue. I find no difference in performance with different octane fuel until pre ignition occurs. Where this really matters is the new motors with knock sensors. Modern ignition systems have the ability to retime each cylinder up to 15 degrees (maybe more on some models)to accomodate the individual and combined needs. In other words if you put low test gas in the car will retard the timing three degrees, each time there is a knock on that cylinder up to fifteen degrees. By driving hard (under load)with low test gas the timing in effect can wind up fifteen degrees retarded over the same car with high test. (within 5 ignition cycles. At 600rpms that would take about one second.) BTW the timing only advances one degree per ignition cycle if there isn't a knock.
Bosch Master, ASE Master, L1
33 years MB technician