Don't do it, you will wreck your cars handling!
Seriously though these cars are extremely sensitive to changes in balance and ride height. If you want it lower do it right with a set of adjustable coilovers and have it professionally set up by a shop that knows these cars well. A quickie, shade tree, DIY spring change often results in nasty handling and bump steer issues.
Driving these cars is supposed to be fun, not a PITA because it skips like a stone all over the place.
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2009 Cayman 2.9L PDK (with a few tweaks)
PCA-GPX Chief Driving Instructor-Ret.
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