KPIs for a comp and benefits person would first have to be tied to the organizational strategy of the organization. For instance, does that organization intend to lead, match, or lag behind in terms of the comp and benefits it chooses to offer.
The C&B person can be measured on things like job evaluation and its relationship to your current compensation scale. Do job grades reflect sound compensation practices? Are benefits administered in a timely and equitable fashion? Are there documented procedures for the relationship between performance appraisals and salary increments?
Are targets set for how the pool of funds reserved for funding new hires is used? The person can be evaluated on their efforts to bring unpaid and overpaid positions more in line with an overall comp plan.
It's hard to answer a question on KPIs for a compensation staffer without having an idea of the performance management and talent management strategies of the organization.