We still have a small number of Adobe ColdFusion 9 (ACF) apps though running on the same server. Gradually migrating from one engine to the other on the same machine has worked quite well for us.
FusionReactor is also installed for monitoring ACF, but I only recently got round to setting up an instance for Lucee. Comparing the System Metrics for the two engines side-by-side, one number jumped out at me: the Average Request Time.
Having run for the same amount of time the figure for each engine was as follows:
Granted, not everything was equal. The type of work each is doing is broadly similar, but Lucee is powering more of our apps than ACF and has a larger portion of available RAM assigned to the JVM. It's also running on Java 8, with ACF still on Java 7.
But Lucee also handled more requests in the same period so the load is higher.
Given the amount of effort required in terms of code migration, it's pleasing to see a concrete reward in performance, although that certainly hasn't been the only improvement.