Considering its not an FPS hit on the games I previously mentioned that had it, I dont believe FPS is really an issue. I do think that possibly the expectation may have been to model vapor in a more realistic fashion, which there are so many variables for that to be done that it would be nearly impossible to do.
If they could do a simple implementation of vapors like they did in previous titles I mentioned but maybe make it a little more visually pleasing textures.... I think it would be great.
Stangpilot.