Who said the grace could outrun the P-51? Its
maximum speed is like 360 mph at altitude, on the deck its much slower.
The point ergRTC, is that the Grace would like a Kate on steroids. Very good turner, great acceleration, and good guns, plus the ability to carry that all important torp.

Somewhat like a zeke with a torp.
So basically you get to carry that torpedo, and then fight off that never ending horde of Seafires that tend to swarm around the CV's in AH. It would be a very fun and useful MA torpedo plane.
For scenario's the Judy and the Jill would be much preferable over the Kate and the Val, since they fit in better with the rest of the midwar to latewar planeset we currently have for the PTO.
If you want to go early war, with the Kate and Val, we would need too many fighters we don't have, such as the early zeke, Ki43, F4F, P40, P38F, and the P39.