My wife works at a dealership and she said the extended warranties save people all the time, but that's on Honda's. Not on a heavily modded Mustang.
Sooo... I did end up buying one and I'm now trying to decide if I should cancel it and get around $900 bucks back for more mods
or just keep it for the possibility that it would come in handy later.
Your dealership should be on your side when they make claims for aftermarket warranties, but if they ever sent anyone out, i'd probably be denied.