Mudd, I even run 170mm cranks on my trail Nomad because of pedal smacking. I can only imagine what a really long travel DH rig like V10 would do with 175mm cranks. It would be interesting to see a V10 with the suspension completely compressed while sitting on a flat piece of ground. The ultimate BB height would probably be scary looking. This isn't a slam to the V10, as most long travel DH bikes would exhibit this same characteristic. Otherwise you'd start off with fully extended BB heights of 20" or such...LOL!...OK, a slight exaggeration there.
On the difference between 170mm and 175mm cranks on a trail bike, I was surprised to find that there wasn't a night and day performance difference in pedaling power and efficiency. On long travel bikes, perhaps it's not as noticeable. Short travel bikes, rigids, and other such bikes seem to be more sensitive to crank length as far as power delivery is concerned. You spin the cranks on long travel bikes more than just "mash" them, so maybe that's the main issue. Riders are different, however, but this is generally what I notice as far as crank length advantages. In the long run, however, seriously smacking pedals on long travel DH bikes is probably the biggest reason to run shorter cranks. Everything's a compromise.