What youve got to remember is if you drop it on springs more than likely you will have to change the dampers after about 12months as they start to bottom out and give way.... so that £15/£30 will be more like £150/£200.... Ok its not so bad lowering it 40mm but they can still wear out after time!!!
When i had my clio with 17's on, i enquired to fred on here (the lowering god) and he told me it was possible to go 80mm all round and 100mm if i had the right offset...maybe needing a little grinding off the arches which he would see to...so personally i'd save up some cash and put it on its tits,,,,
Kiza