I think moe should change the grading.

Make average B+ instead of following civil service grade c as average.

New performance grade.

Excellent A+
Outstanding A
Above average A-
Average B+
Slightly below average B
Below average B-
Needs remediation C+


Of course the bonus don't have to change. But it looks better and feel better for the teacher

In universities we have grades inflation. How many honours are we dishing out. In army rank inflation, wtf now 3sg is nco? By right the sect com is just corporal. But to modernise army career, Corporal is inflated to 3sg. The warrant officer inflation is also crazy. Making everybody warrant officer. in bank job title inflation. How avp you see.
But these superficial things improve morale. And doesn't increase cost.

So I think we could easily change the grade of performance by inflating it， to improve morale, at zero increase in cost.