When my 2007 G35S was installed KW V3 and only stiffer rear sway bar. One year later my front tires were shot because I drove it fast and the front sway bar was too weak. After I upgrade front sway bar, my front tires never have problems again.
If you drive you car fast, it would better to upgrade your car. Both you and your car feel happier.

Back to your question, do you need stiffer sway bars. It depends on how fast you drive your car during cornering. If you feel too much body roll, you will need it. I think only you know the answer.

Some body roll is not a bad thing because it will tell you how fast your car is taking a corner currently. In addition, too stiff sway bars will also reduce your traction which is very dangerous in rainy day .
Civic is FWD. If you install more stiffer rear sway bar, it will reduce under-steering a little bit and you will feel more balance during curving mountain roads.
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