Revert to a 17" wheel (someone here will confirm this is OK for most models) and adopt a higher profile tyre ie /55 vs maybe /45, this will add to comfort and improved ground clearance. A 'modest' driving style should then deliver the required improvements and a slightly inaccurate speedometer - and check it's all OK with your insurers.
As to making changes to springs, I'm not prepare to comment but others on here are expert on the subject.
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