Decided to bit the bullet and replace the OEM Michelins with the Hankooks despite the Michelins still having plenty of tread life remaining. Tired of the highway speed vibration, and after wasting time making several unsuccessful attempts to get the problem resolved I'm throwing in the towel. I did not even bother contacting Michelin as I assume there's little they would do for me given the mileage now on the car. One of the local tire stores is a Hankook dealer and they were able to get me the new tires in 1 day. They showed me the inside of both the old and new tires. The Hankooks have foam similar to the Michelins, although it's not exactly the same. The foam on the Michelins did not appear to have become detached, so I don't think that was the cause of the vibration. While I was hoping the new tires would be a bit more more sporty and provide more road feel, but that's not the case, and these are definitely a "grand touring" type tire with more focus on ride comfort than handling. I would say the Hankooks are a bit quieter than the Michelins, but I suspect that's entirely due to comparing brand new tires to somewhat worn used tires. Most importantly, as Yagkemp said, the highway speed vibration is totally gone, and the expressway ride is as it should have been from the beginning. I paid a bit more than $1500 (plus state sales tax) for the Hankooks including road force balancing, and I'm very satisfied thus far. My past experience with Michelin tires has been excellent, but there's something definitely amiss with these. Even the tire dealer that sold me the Hankooks mentioned that Michelin tires are generally very easy to balance. As you can see just on this forum alone the number of complaints is well beyond isolated and goes all the way back to earliest GV60 deliveries in North America. Never had Michelin tires made in Thailand before...not sure how significant that is.