Early SS games were typically set up 5.5-6.0 degrees. Unfortunately, I haven't experimented with EBD so I can't give you an answer on best pitch.
As far as making games harder, every game is different. Meteor, for example, is a game where you want the pitch as high as possible while still feeling like you're playing pinball, not pachinko. EBD and Meteor both have lots of acute angles, with respect to vertical ball movement, so higher pitch makes these games harder. But higher pitch does not always equate to a harder game. Some games have more obtuse angles and a higher pitch reduces the amount of bounce or even alters the trajectory of the ball causing the game to be more predictable, maybe even more controllable. You really just have to experiment to find your best playing experience.