As I see it anything close to even trades are very good for us as we have more cities and are taking his. 13/15 is decent odds for that. With retreats I'm guessing that we will lose a few units more than gj which should be worth it to take the big central city. Even in a bad case we make no headway early and lose a few units.
My singleplayer balance mod of BTS: https://dl.dropbox.com/s/3u6g4b2nfa74qhm...%20mod.odt?