As long as the one end of the spring is parallel, then it should be fine to cut a half coil off, you'll have to fiddle with the spring seats to get everything lined up again, but i think that has to happen anyway when you remove a spring.
If its a progressive spring, with two or three windings that sit really close together, you can probably cut an entire coil off, and not make much difference to the final height.
We were tempted to do it with craigs S4 as we stupidly baught avant rear springs and it sits with a tiny bit of rake, didnt bother in the end as with a full tank of fuel and him sitting in it, it levels out, and gives a bit more height for when carrying 4people!