I have been using the superb context editor for quite a few years now -
http://www.contexteditor.org and it suited me admirably and will continue to do so for as long as it works on Windows. Unfortunately, the context editor is no longer supported and was abandoned by the original developer despite it being almost a completed project. It is a superb code editor.
The reason I am drawing attention to it here is that Context was also written in Delphi, is available as a project for further development (the author wants to hand it over to another developer) and it had some advanced functionality that might be adaptable for your source editor and which might prove to be a future upgrade possibility for your existing users.
The look and feel of Context is slightly better with nicer icons &c, it also has a more slick feel overall. I have tried both editors and I would convert to your source editor if it had some of the functionality and options that are available in Context, they are both remarkably similar and there seems to be a lot of commonality that might be usable in your offering.
Hope this is of use to you.