IMO a 2.5mm change in crank length is not perceptible. A 5mm change is not really perceptible either but shorter cranks raise the effective gear ratios of the entire drivetrain. The combination of shortening cranks AND increasing the size of the chainwheel two teeth may well be a bridge too far. Especially at the bottom end. True, the o.p. is mainly concerned with the top end. Aren't we all. However, as per OEM spec, the 40x42 is an attempt to give the rider a little bit of an underdrive. Not a terrible thing. With a 1:1 42x42 the bottom gear becomes the rear wheel diameter, 28". That's kind of high, well it is for me! With 170mm cranks to spin it, it might just work. With 165mm or less, it will be that much harder. Too much so? Probably. But 90% of bikes sold in the US are overgeared out of the box. Mostly no one complains. I'll go away now.