The difference between an APP who transforms a practice and one who is expensively underused is almost always design, not the individual.
Design the role, then hire
Write the visit-type list first. Which follow-ups, which medication management, which catheter and post-operative care, which procedures within scope, which acute-access slots. A role defined as "sees whatever the physicians cannot" produces an unpredictable schedule and an unmeasurable contribution.
Verify scope and supervision for your state
Scope of practice, supervision requirements and collaborative-practice agreements vary substantially by state and by licence type, and payer policy can add requirements on top. These determine what the APP may perform and how it may be billed. Confirm the current rules that apply to your practice — this is a compliance question with revenue consequences, and it is not one to settle from a conference talk.
Get the billing model documented
The available billing arrangements carry specific documentation, supervision and location requirements. Whichever model you use, the requirements must be met contemporaneously and be auditable. Practices are far more often caught by inadequate documentation of a permissible arrangement than by using an impermissible one.
Measure the right thing
The APP's contribution is not only their own collections. It includes the physician capacity released — the procedural and new-patient sessions that became available because routine follow-up moved. Measure both, or you will undervalue a well-designed role.