Hello Franz,
You need to check what exactly the user is trying to do. Without knowing it and proper understanding its not suggested to increase any parameter.
As per the information provided by you, you have assigned 24 GB to the SQL and the parameter em/initial_size_MB is set to 20 GB. By looking at all this it clear that you dont have any space for increasing any parameter until you add some RAM to it or else based on the performance history you can reduce the size of EM is its not optimally used and then increase few other buffers.
First thing you can do it try to get the clear information on what the user is trying to do.
Regards,
Nirmal.K