I somewhat agree with you but not with how absolute you are stating it. ROMs do affect performance, in many ways. Fewer screen elements, less to render. Remove debugging/logging code. Just two off the top of my head (I'm not a dev and dont' feel like thinking that hard about it). If ROMs lagged the system a lot less people would run them.
Also in my experience there are plenty of stable ROMs if stability is all apps and navigation working normally. ROMs Like CM, Paranoid, etc do typically have some issues since they are much more aggressive about adding new features. It depends on a lot of factors. How long a dev stays with a ROM. What his view is on stability vs features vs performance, etc.
And yes I agree that trying them is the best thing. Especially on a device like the N7 where flashing is so easy and almost any problem can be fixed.