I've recently become interested in GLScene, I'm looking at the oxNewton version, http://www.dave.serveusers.com/oxNewton.html

The demos there include a FPS example.. it's not exactly quake4, but it should give an indication of what GLScene + Newton can do.

I don't know about any speed issues, all of the demos run at a lovely pace on my machine... only slowing down on one demo when I crash my car into hundreds of spheres and boxes which I placed in the scene dynamically.