How can black exploit this lead in development? 1.e4 e5 2.Nf3 Nc6 3.Bc4 Bc5 4.d3 Nf6 5.Ng5 O-O 6.Bxf7 Rxf7 7.Nxf7 Kxf7

I understand that black should aim for d7-d5 to open up the game, but what else? Are piece exchanges good or bad? Can I get a winning endgame? How and where to attack?

I understand the theory for positions like these, but the advantage always just fizzles out into nothing for me.
I don't see why you should go for d5. Generally speaking opening lines is beneficial for the side with the rooks. And your pieces will get or already occupy nice active squares without d5. I'm not saying it is necessarily bad, but it strikes me as a plan that can definitely backfire. Just imagine you have an isolated pawn on e5 and white's rooks are lined up on the e-file ...

Right now you are up material and you have better development. I would just get my king back to safety with Kg8 and then proceed to develop my pieces (d6,Be6,Rf8, maybe Qe8-g6). My longterm plan would be to build an attack against the enemy king, because rooks are bad defenders and you will be able to have a more pieces in the attack than he can bring into the defence. 

Going for an endgame entails a certain amount of risk, because in position where there aren't many pawns left, the rooks gain power. Right now you are significantly better, but if you would take two pieces and three pawns and maybe the queens away from the board for both sides, the fight would be much more even.