My IS used to be a nightmare with speed humps but that problem was due to the exhaust - Mr Shipley used to scrape his when he had the Hiper on and lowering springs, I know one day I had a full load in the car, in fact Rob was sat in the middle of the seat, and we could hardly drive down a country road as it bowed in the middle and it was seriously scratching the floor :D
As for the car I've got now, don't really tend to get issues with this one, just the exhaust flange catches every now and then but it's not so bad since I had coilovers fitted since they're firmer and don't bounce as much as the shock suspension would do.
At the minute I think this was only dropped by about 20-30mm as I needed to be able to get around and not worry about speed humps (I've been known to turn the car around if there was a big speed hump when I had the IS as I wouldn't get over them without feeling sick at the noise of everything scraping!)