165 Cranks...who runs them?

no - if the old speed was v, the new speed is 1.03 v and the KE changes from 1/2 M v^2 to 1/2 M (1.03 v)^2 = 1/2 M 1.03^2 v^2 = 1/2 M 1.0609 v^2

So, does that mean the 5mm makes a big difference, and we should be worrying about getting the perfect crank length, or can we assume the 5mm is a small distance so the difference it'll make will also be small?