Two things affect performance more than anything else, at least outside of memory management issues like Page files etc, and that's polycount, and texture resolution. The Subspace tunnel probably takes up around 400 polys (very rough guess). So four meshes would use roughly 1600 polys, hardly a back-breaker on any system. UT2K3 makes very frequent use of 1024 and 2048 textures, without performance problems on most newer cards.
I wouldn't think it would be a problem unless people got totally carried away.
The 'reflection' will be possible at some point after 3.6.
The moving walls are a little tougher, unless some kind of mathmatical function is applied to the vertices each frame, using interlinking pofs, I would have thought, would create clipping and texture problems

This would, most likely, be considered geomodding, though bobboau has done stuff that jittered vertices about before, so I'm not certain.