Have you tried the 172.5s? I'd be surprised if you really noticed a difference.
I say this because I'm riding on 170s on my road bike and 165s on my touring bike, and my "optimum" is probably even shorter than that (I'm 5'3" with about a 30.5-31" inseam). I looked into swapping out the 170s because I knee myself in the chest when I ride in the drops, but otherwise I have no problems (and with my Campy gear, it'll be some $600 to swap cranks!).
Then again, I'm a pretty beginner triathlon participant. I won't even call myself a triathlete, so take what I say with a grain of salt.