A great codebase to test RubyMine on would be Canvas LMS. It's a fully open source Rails application used by many universities around the world. I work on it at Instructure, where I and many others use RubyMine. The codebase is large and complex and RubyMine frequently feels slow on it and crashes occasionally.
I love RubyMine, but Canvas seems to push its limits and could be good for performance profiling, if you aren't aware of it.
Github repo: https://github.com/instructure/canvas-lms
Others may want to add their suggestions too.