As far as performance and compatibility goes, keep in mind two things:
1) As far as compatibility goes, the first cards to support shader model 1.1 (not sure about OpenGL equivalent) were the Radeon 8500 and the GeForce 3 (non-MX version). These days it is much less likely for someone to NOT have shading capabilities in their cards (in general - maybe the planeshift population happens to have lower end systems?). And you can always use a non-shading backup. Doesn't CS have an "effect" model now that allows you to define an effect, with backup methods where the preferred method is incompatible (or if the user has a setting to turn it off), and apply them to maps? It's been so long since I've had a look at the code. I'm guessing you'd actually want to target shader model 2 or greater (Any directx 9 card or OpenGL 1.5 I think).
2) As far as performance goes, not all shaders are designed for glowy/shiny/bumpy effects that you can see. Some shaders actually improve performance by taking some load off the CPU. There are three main areas I could see shaders being used to improve performance in planeshift without any real visible change: character animation, the terrain engine (maybe - it's pretty good already), and animated ground clutter (grass, trees, etc..). But, of course some of these things would need to have support in CS first, if they don't already. I think Cal3D at least has hardware skinning support doesn't it?
