Lowering springs generally do only 1 thing, lower the car. Sometimes they're harder too which makes the car corner flatter at a cost to ride comfort. You pays yer money and takes yer pick as they say, you won't get to know what you've bought 'cos the manufacturers normally won't tell you, even when asked. Cars generally do pass mot's with mildly cut springs, mine do. A cut spring is both lower AND harder, which is what most people generally want. And unless the cut springs part company from the mountings (which is too easy to do on the back of a micra) they are usually safe. Best bet if you don't know what your doing or cant get expert help is to buy a set, £90 gives you a choice of brands and pretty colours too. Removal of the rubbers will lower the car and it wont drop a spring but there's no point 'cos you can't drop the front to match. Don't faf around with 30mm drop, go straight for 50 or 60 it's only the length of your finger and it won't spoil the cars capabilities on the rough - even the rally micras run well below stock ride height.