Siemens ECU is "self-learning". It uses a closed-loop mode below 3000 rpm, if I'm not mistaken. In closed-loop mode, it will dynamically adjust the fuel injector pulse duty cycle based on the readings of the lambda sensor, airflow sensor, etc. to maintain cleaner emissions. So, if you introduce some device to fool it into running richer or leaner, like the SAFC, the Siemens ECU will just compensate for it based on what it picks up at the sensors. You'll just end up with no gain. For instance, if you intend to use the SAFC to make the engine run richer for better acceleration, the Siemens ECU will detect from the lambda sensor that you're running to rich, and it will just reduce the fuel injector pulse duration to lean out the mixture.
For the MMC ECUs, they're using fixed maps, which can be fooled by the SAFC and other such piggybacks.