The answer is really simple.
It does void warranty.
Everything that is not original factory part in engine bay voids warranty. Even if it has nothing to do with (for example engine blow up).
But a lot decisions are made by the dealers only, so some might look things a lot through fingers and some not. Thats the only difference.
Tip: if you service your Seat @ seat dealer and you want to keep the warranty on, uninstall enginebay mods before, because they will take photo on each maintenance especially if there is something in there that does not belong.
Ref: my friend lost a battle of a blown engine with only 28000km on the clock.
He had installed forge dump valve and thats it. NO WARRANTY, because engine modded.