I fitted a set of springs off a Fiat 500 to the rear of mine.
Lowered it around 15-20mm, but more importantly, they're slightly softer, so makes the ride a little less jumpy.
Combined with Koni shocks all the way around, it goes round bends like it's on rails, and no issues with speed bumps.
Only time I've ever had an issue, was when I was driving down to Spain fully loaded. Car was packed to the roof inside, and 3 mountain bikes on the back. On a bumpy bit of road through the Pyrenees, it scraped the exhaust a couple of times, but fine apart from that.