I agree 100%. The vast majority of pond owners I know, myself included, could care less about their koi being "show quality" or not. As long as the koi look good to them, as long as their happy, then that's all that matters. A very small percentage of pond owners actually show their koi, which is also worth mentioning because it would be easy to get the wrong impression from these forums where everyone's talking koi shows all the time.
I'm sure there are plenty of quality domestic koi over 30" in the states. Just because they aren't shown doesn't mean they don't exist. Another issue to keep in mind is the fact that the term "quality" is subjective because everyone has their own opinions. A great koi to one person may seem less appealing to someone else. It's all a matter of taste.
If by quality you mean whether anyone has seen a domestic jumbo koi capable of winning a koi show with Japanese standards, then I'm fairly certain the answer is "no." But again, Japanese standards are not necessarily the standards used by the average pond owner when it comes to choosing the fish to add to thier ponds.
Sorry to ramble on. Have a great new year everyone...