Hi, a fair few people use SPAX lowering springs here in the UK.
http://www.spax.co.uk/start.php?type=modern&lang=en
They are a decent brand, not too crashy or stiff, but not
progressively wound like eibachs etc, hence they are probably
half the price of the eibachs.
If your using them with standard dampers I wouldnt
go any lower than -40mm as a general rule
Sam.