demon tweeks
Go to demon tweeks, (or some other good motorsport distributor). You can buy springs of almost any size and compound, just find out what diameter you need and the standard height and compund and they will tell you what to buy.
I've had mine done, just 5mm lower but stiffend up a bit to get round the twisty bits better. I would also reccomend Leda adjustable shocks, very good but a bit pricey.
Hope this is useful