In addition to installing aftermarket springs for people, I also lower cars by
precisely measuring and cutting stock springs. Based on this experience, 1.5" works well as a street lowering for cars I have worked on. More than 1.5" requires a camber kit for most cars I have worked on, or the negative camber eats the tires.
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I find that good shocks keep a car from bottoming, and stiff springs cause a car to lose grip in the turns, a lot of times because the stiffness causes the car/tires to bounce around, instead of following the road contours. Negative camber on the cars I have worked on will cause reduced grip due to less tire contact in the turns.
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If you are planning to really slam it down, you might have to get both shocks and springs from the same manufacturer. When some MFG's
claim "X" amount of lowering, it can only be achieved by using their spring/shock combination.
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There are several things to be measured *in addition to* tire/wheel well clearance. A spring that's too short for your particular suspension is
too short whether you paid $300 for it, or made it too short all by yourself........................something that not everyone on this site understands.

And a spring that's too short, no matter how you got it too short, is what flops around loose, eats tires, and beats up your suspension.
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Some people have me precisely measure and cut their stock springs while saving up for expensive aftermarket springs, and I am the only person I know who is able to precisely measure, to avoid the above problems.
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My reply is based on my experiences, and not internet "research"
of crappy car repairs. If you want to verify that all I say is true, you can ride with me sometime.-Bob