For the n-th time:
a) No, a couple of thousand!! more polygons won't make a difference for engine, as it was said for a million times, it's the size and number of textures that's the real killer. (Actually raw geomertry rendering is expected to be even faster in the new versions of the engine, since according to taylor with the end of D3D support he can finally optimize the code).
b) Yes, it's actually visible, unlike retail FS, or some other old spaceshooters (or even new ones) out there, the lighting in FS_SCP really brings out the details. All those ridges, bumbs and general greebles come to life when the shadows/lights of dynamic lighting start their magic.
c) No, the modelers don't have to start makeing low poly models - actually they alredy DO. That's what LODs are there for. If your puter can't handle it, you can use the darn settings.