I'm upgrading to an i9-13900K + 4090...whats fps can I expect in VR?

A simple benchmark (CPU-Z) tells me your 10900K is about 3-4% faster than my 5800X3D.
Your 4090 is probably significantly more capable than my 3090 Ti.

I saw an overall increase in FPS (about 10-15%) when I upgraded my 5800X to the X3D.
Demanding object environments are tough and my FPS bump in those environments are less. Don’t expect the world.