so my commencal supreme DH has adjustable chainstays (about 1″ of adjustment).

whats the pro-cons of longer/shorter?

I know

shorter chainstays = easier to manual.
longer wheelbase = more stable at speed.

what else?

longer cs = a bit more travel? will also add a little more weight to the front end for a given rider position, may help cornering in some situations as the back will drift out easier. well based on my exp of rigid frames with different CS lengths. i’m guessing you don’t care about the effect on climbing though )