I'm not sure any manufacturer yet has a way to accurately and consistently grade a bat on performance. If any batmaker is, ask them how they're doing it.
In my view the only way to grade a bat is by using things which can be measured accurately and objectively. That's essentially looks or cleft weight (when dried to the same moisture %).
Perhaps one day a widely available test will exist for bat performance. If that can be done at the point of manufacture then it is only at that time grading by performance should be adopted.