Let's take a simple look at the issue at hand. Assume you simply want to lower your car by 25mm. If you replace your existing springs with an identical set of the same spring rate, but which are 25mm shorter in free length, then your car will sit 25mm lower, plus or minus a mm or so. If you also want stiffer springs, you obviously need a set with a higher spring rate, which is a measurement of how much the spring compresses for a given force, often measured in pounds per inch. But now you need an even shorter spring to get that same 25mm lower ride height because the stiffer springs will not sag as much as standard rate springs.
Furthermore, many coil springs these days aren't bent and ground flat on the ends. They fit into specially molded rubber spring perches so the ends must be the correct shape and the cut ends must be indexed properly too if the upper and lower spring perches won't rotate relative to onea another.
And, if your springs fit over a shock absorber, then they must obviously be large enough to do so but not so large in diameter that they clash against some other part of your suspension system. Finally, if you use springs from a car of a different weight or different weight distribution, you have to consider how the different spring rate(s) will effect your car.
You can consider all these factors or you can simply try to fit the springs and see what happens. Or, you can forget all these potential headaches and buy a recommended set from a reputable after market suspension supplier. Suit yourself. But if you do actually use the springs in question, report back with your results please.
