All I know is 160mm cranks with a ratio of 18:13 and a light chain, will make the drive train feel soo nice for gaps, sidehops, static backwheeling and pinching which really helps when riding natural and rocks!
Longer cranks like 170/5mm are good for leverage power, but are not too good on a mod for anything except sidehops, sidehops and sidehops and the odd gap. Also I think that longer cranks on a mod make you roll more when on backwheel. Meaning you have to take off further back when gapping(IMO). Just think, manufactors don't just shove 160mm cranks on mods for no reason, they have tested to find the best length.
I would stick with the 160mm and get a 13T rear sprocket. Also I would get some nice Trialtech cranks!
Nathan