An enterprise server, both with and without mainframe subsystem support, has a number of diagnostics that can be active during run time.
Whilst it is true that an application configured with diagnostics switched off will be more optimized, it may also be of benefit to run with some diagnostics switched on. This is so that unexpected problems can be investigated when they first occur. Diagnostics consume resources, and there is always contention between problem determination facilities and performance. You need to decide how important it is to be able to diagnose a problem the first time it occurs rather than having to try to recreate it or having to just turn on diagnostics and wait for it to re-occur.
There is a recommended strategy for setting the right level of diagnostics detailed in the Enterprise and COBOL Troubleshooting Enablement Pack, which can be downloaded from the OpenText Support for Micro Focus Products website at https://supportline.microfocus.com/examplesandutilities/index.aspx. Follow the details set out in this guide, and then tweak them for your specific needs.
As well as deciding on the level of diagnostics that will be active, you also need to consider the actual workload expected of the server; there are a number of factors to consider; see Performance Considerations.