With knee pain developing on my LWB recumbent, I went from the stock 175's down to 152's with appropriate gearing down of the front chain rings.
The knee pain went right away, and I'm as fast (or maybe as slow) as I always was and climb seems unchanged.
I think the shorter cranks make your knees flex and extend less, and while shorter cranks DO result in decreased leverage, with appropriate gearing you won't notice any difference in speed or hill climbing, etc. Unless you are Lance Armstrong or some young jpck or speedster.
I doubt 5 or even 10 mm change would be noticed at all.
When you think of it, we all require different size frames, handlebars, seat heights, so why do we all accept 170 to 175 one size fits all cranks. But most of it is trial and error anyway - if it feels good, do it. What works well for one guy might make the next guy's knees hurt.