I have some experience with moving sprite with touch and it was mostly a pain because I'm too used to mouse and keyboard.
Anyways in my experience 8 directional is not good. Just use simple MoveTo behavior and set it like this:
-> Touch - is in touch: Player MoveTo Touch.X, Touch.Y
Change the MoveTo settings to make it go faster/slower.
And use ScrollTo like winkr7 said so that your player is always centered. Scroll to doesn't move any objects or backgrounds, it works like a camera movement.