I got the cat carcass back from the taxidermist and got a good look at the trajectory and ballistic damage. The cat isn't displayed quite right and the yellow line really just indicates the areas the bullet passed through, but you can get a good idea how much damage the 6.5 Grendel Hornady SST 123 gr. bullet did. It was probably still traveling 2500 fps at the time of impact.
The bullet passed through the left arm above the elbow, breaking the humerus at that point. It entered the chest cavity and broke up a lot of the rib chest plate and wrecked the heart and lungs. It finally exited the right side lower ribs with a hole about the size of a ping pong ball.
The damage seems about par for the course in terms of performance of this round. It tends to break up when it hits hard tissue and I think this is what the damage is reflecting. I really like this round for hogs, but it isn't quite so ideal for cats.
It will be 2-3 months before the tanned skin gets back to see how much damage was really done to it, but I think the exit hole may be a problem.