I have never understood the fascination with small changes in crank arm length. In addition I have yet to see an empirical study that shows an advantage, have seen several that show no advantage, and even one person (http://www.powercranks.com/cld.html) who shows evidence that the only change that makes sense is to go much shorter. In no case do I believe from the evidence that 2.5mm is going to make enough difference to justify the expense and trouble.

Longer cranks have never made sense to me, as they increase the bend in one's knee at the top of the stroke by the same amount as lowering one's seat by double the crank length change. It is well documented that knee stress is increased as the bend in the knee is greater.