You'll feel when it goes into LOS, poor throttle response and very down on power. If it drops into LOS when you're on boost then you will suffer from severe detonation, you'll be able to hear it from in the car. If the ECU detects a malfunction of a major sensor, e.g. MAP or TPS then it fixes the ignition timing and injector pulse time. This suits a warm engine and medium load but in pretty much every other condition it will run very rich. Fuel economy will become dire. Other sensor failure results in a substitution figure being used and no LOS.

I'm not sure what voltage the output from the stock MAP sensor is or what frequency range it generates. It might be that the Arduino is outputting too low a voltage on the frequency signal to the ECU.

It would be so much simpler if Ford had used the more common 0-5 V output MAP sensor. As I said, using a vacuum switch and valve to stop the boost signal getting to the stock MAP sensor is theoretically simple but getting a quick enough response would be the challenge. I did try changing the switching value of the vacuum switch but it didn't help.