See,
http://www.cranklength.info for all you ever wanted to know on the subject. If 170mm is the right length for you in the first place then 2.5mm is neither here nor there however if your legs are on the shorter side them moving longer may be detrimental. Going longer may make it harder to be comfortable in an aerodynamic tuck which will wipe out any benifit you may beleive that you will gain.
Regards, Anthony