if your going to lower it, i would not go any lower than lows, But I would not get kings springs, they are WAY to bouncy, superlows are not as bad, but I'd get lovell's low springs.

I've got kings superlows in mine with the EL Tickford upper trailing arms, and with out the diff bump stop the tailshaft was hitting the floor very easily even with just me in the car.
I scrape on heaps of speed humps with just one passenger.
I've got the springs matched with Koni reds all round and I find it too bouncy. My XF with kings superlows and koni reds handled and felt better on the road.
I am finding the superlows just too much of an inconvenience for so many things, car ramps for servicing, trying to get a trolley jack under the diff, driveways, etc.

I am going to get some dobinsons lows (comfort option). Kings are just harsh and nasty.