have you ever rode a bike with 165 cranks?

i would stick with what you know.

165mm feel so small. you do notice it.

I'm just under six foot and have an inside leg of about 32". I run 165mm on my DH bike and have run the same on my trail bike until very recently, when I went back to 175mm simply because that's what the LBS had in stock when I needed a set.

I honestly can't feel the difference but then tbh I have little idea as to what the difference is likely to feel like.

On a DH bike, the benefit is going to really depend on how low your BB is. If it's one of the new Spesh Demo 8s then they have somthing like a sub 13" BB which is lower than haydes so yes, probably 165mm are needed regardless of how tall you are. If it's a much higher BB then maybe you won't feel any benefit.

It will also make a difference as to whether you run flats or Spuds. If you ground out on flats at speed, then you end up going down really hard. On Spuds you can ground out and get away with it. So if you're running flats, then perhaps having 165mm cranks is a good insurance policy?

I think it would make a difference to the pro/elite level XC field. I am not sure you'd feel at a disadvantage on a DH bike unless you were on a super pedally course.

I suppose, taking a purely mechanical perspective, shorter cranks generate less mechanical leverage. You would most likely notice that when really cranking onf a big gear. On a DH bike, you should aim to be spinning the pedals at a smooth rate not cranking a massive gear really slowly so I think, mechanically, there is no advantage to running longer cranks and only disadvantages.

BBs seem to be getting lower, just like handlebar rises and front ends. The sub 13" on the new Demo 8 is ridiculously low (although it's also relative to how much travel you have of course). 14-14.5" on an 8" travel bike used to be pretty standard and anything less than 14" was regarded as being low.