have struggled with the same issue - what KPI for Accounting department make sense? A KPI should do two things -- (1) inform management and employees about the efficient operation of the company/department and (2) ensure employee behavior is aligned with the company goals.
If your department is having issues with the work flow for AP or AR then your choice of metrics may make sense. However, you want to make sure that tracking the KPIs is going to lead to an improved operation. Selecting the target for the KPI is critical as well. If it is too easy or too much of a stretch it loses its value.
We opted for measuring results only through AP and AR aging -- setting the acceptable levels and then managing to them. We stayed away from the targeting of the number of AJEs, since many of the adjustments are caused by errors in other parts of the organization that Accounting needs to fix. I would not want to have an accountant pass on an AJE if they were afraid it would hurt their KPI. We did track days to close and the completion the closing calendar on time.
The overarching concern of a KPI should be to balance the measure against the benefits of having the measure in place. Keep the KPI target/goal realistic. And, be careful of unintended negative consequences.