Idea: Create a in game DPS meter to allow see all party members DPS. Then, add a option in the character configuration window saying: "Allow other member of the party see my DPS" If you marked it, others can se it, if you unmarked it, it is private and others cannot see your DPS.

If people start abuse the DPS meter, more people will deactivate it. If people does not abuse, probably more people will activate it. I think this can work.