I use unlimited as well because I have well over 200 of my own Sims and I keep making more, lol. The game still generates random townies so every so often I go through and delete the ones I don't like and clean up the keepers a bit, give them a theme.
I think the max Sim count might depend on how many EPs you have, but I'm not sure. I think they boost the number with each one.
So far I haven't had any performance issues that I've noticed, but I kind of expect to at some point, especially if my Sims start reproducing after the romance pack comes out. :D