I still prefer to go with coils that are specifically manufactured to be shorter. Also, most OEM springs aren't progressive, so if you go with an aftermarket replacement that is progressive (such as the Eibachs listed above), you might be able to lower your car while enjoying an improved ride quality.
The other thing you'll have to account for is the shock/damper travel/length. If you go too short, you risk destroying the shock/damper, or at the very least, decreasing its lifespan significantly.
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