Metrics like training data set size are less interesting now given the utility of smaller synthetic data sets.
Once AI tech is more diffused to factory automation, robotics, educational systems, scientific discovery tools, etc., then we could measure efficiency gains.
My personal metric for the next 5 to 10 years: the US national debt and interest payments are perhaps increasing exponentially and since nothing will change politically to change this, exponential AI capability growth will either juice-up productivity enough to save us economically, or it won’t.