First off, let me say that anyone that claims their server's CPU is struggling on a standard 6 player game is just full of it. We've gotten feedback from server hosting companies running hundreds of KF servers as well as other game's servers. The feedback we've gotten is that the CPU usage of stock KF is actually less than that of L4d. With even low end CPUs you can run several instances of KF per core. So CPU usage for stock KF is not an issue for servers.
Regarding adding the ability to add more zeds per player, this is something that we are considering. There are a couple of issues here though to think about. One is gameplay balance. Adding more zeds per player throws off the gameplay balance and perks earning, and will require extensive beta testing at all different levels (32 per player, 50, etc) to ensure the game is still balanced and fun. I know someone will reply to this thread and say "we don't care if its balanced or fun" and well, sorry, but when you are in the business of selling games you have to care.
Regarding CPU usage for increased monster counts/player counts, the Unreal 2.5 is what it is. We're not going to invest massive man months (or years) worth of effort to take the engine multi core, nor are we going to rewrite Epic's base AI code from scratch (which is what would be required to optimize it more). Epic's AI code in 2.5 is pretty heavyweight. That is just a fact of using the engine.
We designed the base game to run well on a broad range of machines, and we accomplished this goal. We also gave the ability for people to go crazy in sandbox mode and run whatever they want.
So to sum up, we're looking at opening up some increases in zombie counts for the standard gametype, but don't have any sweeping changes planned to go crazy like 80, 100, 1000 zeds at once.