The Mamiya 6 is a great camera but not what I'd use for panoramics.
If your goal is to shoot panos, then get a camera that shoots panos...simple as that.
Like Tim, I've never shot with the grad filter on my XPans and never had a problem. Guess I'm 'insensitive' too!
It seems your goal is to make panoramic images, and I'd keep the XPan for that. The drawbacks are pretty minor compared to those of the Mamiya for this purpose.