Spax are a very hard ride with not much give in them at all, Koni's will be your best bet mate, adjustable's would be better as you can obviously adjust the ride to suit how you like it.

A 40mm drop on 15's will be very low, i'd usually say only go to about a 25mm drop at the most, too much and the RST's don't really respond to much from it

Get some Cobra springs from APT, i'll be getting them asap and just sticking with a 25mm drop, but i've got a Series 1