Can you really have "excessive empathy"?
The risks of excessive empathy is sometimes conceptualized as "compassion fatigue" or "vicarious traumatization". The problem is not necessarily considered an excess in empathy, it can also be described as an excess in work load. 

What could be considered excessive is most likely individual, not unlike our capacity for handling other types of work, like lifting heavy loads or work long hours at the office, for instance. 

The most reliable measurement i can think of is how much time a day we spend working as a therapist, for lack of better constructs. Another example for conceptualizing excessive empathy would be the prevalence of fatigue/secondary ptsd symptoms.

Also i'd be curious to see how therapists ambitions/expectations on therapy outcome factors into the risk of compassion fatigue. I don't believe such research has been made.

More research is needed for sure.

https://proqol.org/CS_and_CF.html