Nah, 12,000 polygons is no sweat for a computer nowadays. It's like the difference between moving your finger 1mm or 12mm. So unless you're trying to play this on like a PS2 or something, I wouldn't worry too much about the polygon count. Now (to paraphrase MM) if some modder made a 12,000 polygon coffeemaker and some genius decided to build 500 of them in his base, yeah, that'd be trouble. But under normal circumstances there are only a a few screamers around at a time.
There *are* performance bottlenecks regarding how Unity handles things, but I don't really understand the particulars. They've talked about one of them in this thread: "something something texture streaming something something memory something" was about all I got out of it, though. Would be interested to learn more if someone felt like making an effortpost about it.