Well, I can certainly confirm that entity changes work.. I've had no trouble adding new spawn points and such, and that causes no noticeable lag. In fact, I believe that cleaning out the unused/nonfunctional SP entities actually improves load times. I also don't believe prediction comes into play for entities that are not tied to brushes, since those are always taken from the server, as far as I know.
I'd never heard of a game allowing these kinds of geometry changes though. I will give it a shot and let you know how things work out.