Basically i have an adjustable traction arm on my Altezza, and ill be damned if i can figure what effect it will have to the handling if i lenthen or shorten it. All things being equal i would be moving the dave point front or back...but what effect does this really have on the rear? Also what should it be set at idealy if it doesent do much? (yes they are megan and yes they are crap...ill replace them later)

It's just one more point of adjustment, it should effect camber, caster and toe. I would leave it's adjustment to a qualified tech with an alignment machine.

The adjustment affects mostly camber and toe with a little roll steer and antiquat as well.

so there is no real advantage or shortening or lenthening this arm all other things being equal?

There might be but its hard to know for sure from a picture. I would have to string it up and measure some things