5mm makes a big difference on recumbents where the leg length is locked in. On a regular bike, the body can compensate for slight differences by moving back and forth on the saddle which a recumbent rider can't do. I run 165s on my bents and can tell quite a bit of difference if I put on 170s, mainly because I'm shorter than average (5'7"). With 170s, my knees come back too far causing an over-contraction and I get sore thighs. FWIW, I've got a line on a set of 155 mm carbon cranks and might give them a try.