We all know that manufacturers leave a margin of error for safety but yes, any increase in power using a part not offered can be grounds for warranty voiding. All they have to say is you increased the HP level over the stock levels which strained the parts beyond their designed tolerance. Also, because of the extra HP the engine is making the trnasmission is stressed more as well.
Do I think ANY tune on an NA motor has enough of an increase in HP to be a problem? Not even a little. But as PG said, if the rev limiter was raised then you're screwed. But push comes to shove, you modded the engine and you have to live with the consequences. I mod all of my cars and I accept that some of what I do may void the warranty on some things.
Ya gotta pay to play.