It's made by Getrag, and I've heard they're having issues with about 2-3% of the manuals out there. That's actually typical of new vehicles, this is an entirely brand new transmission in an entirely brand new engine, so it's bound to have issues.
If I had to guess, I'd bet it's similar to why a lot of folks have issues with TREMECs, probably a slight misalignment problem with the input shaft due to either manufacturing tolerance in the blocks, bellhousings, transmissions or a combination of the 3.
It could also be a material spec issue with transmission components. Currently manufacturers pretty much have to spec the lightest materials and make the components just strong enough for the application in order to keep drivetrain efficiency up for mileage purposes. Occasionally due to variances in manufacturing, it results in parts that warp etc.
Who cares how much horsepower it has, all that matters is how fast it goes!
Untested 331, lots of suspension, chewing up corners.