[Dave928]

for one example, say your cats went bad.

while your car may only be covered by a 2 year factory warranty, catalytic converters are covered for 8 years by federal law (in the US, don't know about Canadia). if you took your out of Porsche warranty - but less than 8-year-old - car into the dealer and they had to replace your cats, there may be an ECU flash update to go with it. they're not going to look at what you have flashed in the ecu, the update installer software is simply going to see that it isn't the current version and do the update. which most likely will wipe what your aftermarket flash programmed.