There is currently no support for filtering the function list in the formula editor. I have forwarded this post to the Developer Support Manager to create a feature request on your behalf.

Hello Johannes, 

I have sent your Product Idea directly to our product management team.  Our product team chooses new Product Ideas for development based on popular feedback from our customer base.  Infragistics continues to monitor application development for all of our products, so as trends appear in requested features, we can plan accordingly. 

We value your input, and our philosophy is to enhance our toolset based on customer feedback.  If your feature is chosen for development, you will be notified at that time.  Your friendly ID for this Product Idea is PI13020026. 

If you would like to follow up on your Product Idea at a later point, you may contact Developer Support management via email.  Please include the reference number of your Product Idea in the subject and body of your email message.  You can reach Developer Support management through the following email address:  dsmanager@infragistics.com 

Thank you for your request.

Hello Johannes,

I am just checking the progress of this issue. Let me know, if I can provide you any further assistance.