Shortening the shocks will not affect the ride height, neither will adjusting their stiffness/hardness. What you need are shorter springs. I doubt if goa will lower it enough to need shorter shocks. To check Jack the car up on a trolley jack in the middle at the back. Take out the springs (put it on axle stands whilst you do that bit) and lower it down until it stops. Don't let the full weight come off the jack, I think you will find it will go low enough for all your needs. In which case either replace the springs with shorter ones, or get some coil overs and then you can adjust the height. Don't forget the front will need similar treatment.