Read here: A guide to consumers about modifying a vehicle & their rights
Basically in theory a modification only voids the warranty in the case of a failure if it can be shown to cause that failure. Which means for example it would be pretty hard for VW to claim that something like a muffler delete caused one of the wheels to fall off or something like that. You can however assume that something like an ECU tune will immediately void your drivetrain warranty, even if it doesn't actually contribute to the failure you'd have a very tough time proving that in court if you wanted to push it.
In the end how much of a problem a warranty claim with modifications would be relies a lot on how generous the dealer is being and what you have done. If they want to screw you they will and there's very little you can do about it, that's generally how individuals vs large companies go in the legal arena.
MY13.5 Polo GTI - Flash red
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