As for DPs, or other free flowing exhaust mods causing over boosting conditions, they certainly can. Even though you're not manipulating wastegates, you're increasing flow and reducing restriction.
Spiking and creeping are both symptoms of turbo back exhaust mods, or cat deleting. As OE tuning isn't optimized by adjusting wastegate duty cycle tables, the gates/control circuit can be be overwhelmed by pressure. This is, of course, a much larger problem with internally regulated turbos. A tuning solution pairs nicely with cat deleting, and often solves the issue.
The best solution for the warranty issue is simply to find the right dealer. I had a buddy technician at an area Mazda dealer, and made friends with the service advisors and manager. The tech would simply "overlook" modifications. I was able to get tons of warranty work done this way. One or more local Mazda community members even had new engines installed, compliments of this service department. Some dealers are friendly. However, it may also hinge on how heavily scrutinized dealers are by the manufacturer. For Mazda, back then, it was simply a tech line they called for support, and it was easy as saying "no/none" when prompted about modifications.
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