The main benefit of using Lazarus was that it made the migration from Borland C++ Builder / Kylix straightforward. It now makes sense to simplify things by using GTK directly. I'm using the GTKMM API that is provided for use from C++ code; it has turned out to be reasonably easy. I now have the simulation statistics based on GTK, and will migrate the rest of the user interface in due course.
Thanks to the Lazarus team. Its a great product and has really helped with the development of SimSoup.