The Air Force must have a good grasp of the KF-21's objective performance.

Because they have the F-16, F-15, and F-35.
They've seen them all, so there's no way they wouldn't know..
I wonder how it would compare if we only looked at air-to-air combat?
It will likely be similar to the F-16V, Rafale, and Eurofighter..
If it surpasses the F-16V, it's a success.