As the scrum master is a mentor and guide for a team, the team trusts the scrum master to lead them on the right path to "being" agile.
I would suggest using this as your guide.
Measure the performance of the Scrum Master by the agility of the team and the effectiveness of their Scrum implementation.
Evaluate the team's:
- Ability to respond to change
- Ability to frequently deliver value
- Ability to inspect and adapt
That evaluation is likely to be a good reflection of the effectiveness of the Scrum Master.
Each team is different and there's no benchmark (Or is there?)
You don't need a benchmark, instead you can track the trends. If it is trending up (i.e. getting better) then the metric is a positive indicator.
Personally, due to this vagueness/lack of clarity in measurability, I find it difficult to decide on SMART goals for myself.
If your organisation absolutely insists on concrete metrics then I would suggest using things like lead time, cycle time, delivered value, etc. You could also regularly survey your stakeholders and see if their appreciation of the team is trending up or down.