Genesis has every right to void a warranty if you make modifications for performance. Any manufacturer does.
Could be an interesting battle. The JB4 (and other tunes) is designed to push the engine beyond the advertised capacity so if you put a valve though a piston or lose a bearing you may have a problem.Whether you want a legal battle to fight or not is up to the individual. But, the United States Magnusson Moss Act (I think) addresses this paraphrasing that a warranty cannot be voided if the aftermarket part did not cause the failure of whatever has failed. That's like having your warranty on your shocks voided because you added aftermarket windshield wipers. The wipers did not cause the shocks to fail. But, again, you might have one hell of a legal battle and spend a lot of money fighting it.