Here we go again.

2.5mm means you can crate 1.5% more moment about the axis with the same force. Not significant. Even if it was, you have to travel a longer distance, so the energy expended is the same. It's like shifting down 0.1 gears.

IMHO, crank length is a matter of fit. If you feel like you are spinning teeny tiny circles, get bigger cranks. (Even then, I doubt I could tell the difference between 170 and 175mm cranks unless I was concentrating really hard.) Otherwise, don't worry about it. It's not going to get you free speed.