Right, I’m sure the ECU is smart enough to make small adjustments, but I’m not sure why anyone would want to take this risk?Modern cars with modern ECUs are a lot more smarter with things like high flow cats and intakes and will make adjustments within a given allowed ranged for more performance. The main thing to take away from this is any performance gain without a tune will not be that noticeable but the noise from the intake or downpipe is enough to persuade more than a few to make the leap.
A stage 1 tune on a completely stock car produces better numbers than a car with a downpipe and no tune. But both cars will be void of warranty in an engine related fail. The smart ECU records high values, and when it adjusts for the downpipe with no tune, the values show higher. An investigation will show both modifications on the logs and both will void your warranty, but one is significantly safer than the other..... so why risk it?
TLDR: Both downpipe and tune change the numbers in the logs and give dealerships a reason to void warranty in the exact same way, so why go with just a downpipe and get little gains with the chance of detonation instead of getting a tune with better, safer fueling that makes more power?