If you go by the 5:1 rule, then 8 strands is plenty. What O.L. and others have found is that D97 breaks at about 50#, not 100+ as advertised. Even so 8 x 50 = 400#/70 = 5.71, so you should be fine.
I have six strands on all but my old Bear (B50 there) and my Mahaska, which has 8 strands of D97 padded to 16 and served with yarn (non-fast flight bow). If the 8 strand wears out, I'm going to six on that one too. The advantages of the skinny strings are reduced/eliminated hand shock, better performance, reduced noise, and $ savings if you twist your own. Disadvantages...well, I suppose if you nicked the string with a broadhead it would break easier, otherwise I can't think of any.