Below is my opinion, based on personal experience, it may be greatly different than others.
I've ridden 170s (with a 38x52 in front, 12x25 10spd rear) on a road bike for "just about forever" (greater than 20 years), 165's on a fixed gear (usually ridden on the road). Stylistically, I've always been higher cadence, I feel on top of a gear in the 95-100 range (made it easy to do group rides with a bunch of track riders). The 165s give you a shorter lever arm from the pedal spindle to the crank axle, so for any given amount of force you apply at the pedal, there is less effective force applied at the crank (simple physics). For a "normal" length of 170mm, the 5mm represents a difference of about 3% in lever arm length; this means that at the crank you'll apply 3% less (going to 165) or 3% more (going to 175). Now, exactly what does that 3% mean on the bike? Normally, about 1 tooth on a chainring. Still using the 170 as the normal length crank arm, a 39x16 with 165s will feel like a 40x16, with 175s it will feel like a 38x16. If you plot out the deltas in gears across the gears normally used by racers, say a 39x53 with a 12x25 10spd, the steps between successive gears is about 1.5-3%.
Climbing I felt the impulse was stronger, but of a shorter duration (a little more pedal loading, but you move through it quicker). Absolute position of the seat relative to the pedal can change the way it feels (seat up/down, back/forwards). It may take a few rides to get used to the feeling, but you should be able to adjust to it. On a mountain bike, you want to limit the delta between peaks of applied power (easier to maintain traction). The longer crankarms mean lower peak force has to be applied, but it's applied for a longer time.
Don't forget that the differences between the GC contenders in a 3 week tour (assuming all undoped) are actually very small. Assuming they all start the tour out with a "full tank", every little bit they save in the first 2 weeks means they have more for the third week. If changing the crankarms means they save a little bit more, then it's easy to see why they run longer cranks.