It seems there not many of us on this forum or we don't post here.. For this question I would ask in the GR sussy thread..

GR-Suspension, Handling, Stiffening - Subaru Impreza WRX STI Forums: IWSTI.com

 

I can't help with suspension but I know if I knew I was going to drive off road often I'd get skid plate and rear diff protector on the car.

I have driven on flat dirt roads with no consequence on my lowered springs. I also once ended up on a road with nothing but big ruts and really only ment for trucks. I managed to navigate through it carefully, but was concerned about bottoming out my oil pan.

 

The stock suspension is probably good enough if the road is in decent shape. If it is really rough then just slow down a bit. I have driven my '11 WRX on gravel roads without a problem and the only thing I have done to it is install mud flaps to protect the paint from rock chips. If you want to get a different tire then go to a 50 or 60 series and that will give you a little more ground clearance.