Hi Rob,
You are absolutely correct. Leonid Elenin has published a graph of Mote Carlo simulations of the impact probability - and it's probably not a lot better than winning Lotto.
http://spaceobs.org/en/
The most likely outcome is a close pass. And that's all.
As the comet sheds surface material during its time nearer the sun, we can expect that objects in Mars orbit may ***possibly*** experience micro impacts as a result. Phit Plait (of BadAstronomy fame) discusses this.
http://www.slate.com/blogs/bad_astro...ober_2014.html
Regards,
Tony Barry