I don't have an "employee development" program. My business is a trade, and the greatest return on investment I ever got was from a simple, hands-on approach: I got my employees to teach each other. It's much better than any formal program.
My process is straightforward. I'll pair a new employee with an experienced crew leader. The crew leader teaches the new employee a specific skill, like how to properly install a piece of flashing. But the "development" is that the teaching is a two-way street. The new employee will often have a question that forces the experienced hand to think about a process in a new way. It's a simple, hands-on way to get my crew to learn from each other.
I measure its impact on individual and organizational performance with a simple, hands-on approach. The individual's performance is measured by how quickly they learn a new skill. The organizational performance is measured by our efficiency and our lack of mistakes. When the employees are all learning from each other, we are a much better and much more resilient business.
My advice to other business owners is to stop looking for a corporate "solution" to your problems. The best way to "measure the impact" of a change is to be a person who is committed to a simple, hands-on solution. The best "metric" is a simple, human one. The best way to build a great business is to be a person who is a good craftsman.