I went with 672 lb. rear springs and very happy with them. Not too harsh with proper damper settings. Actually rides much better than OEM sports package. You could always keep the front kit springs and just replace the rear springs if you are concerned about the rear spring rates.
11.5 @123 MPH on PSS | JB4 G5 w/WEDGE flash | FBO | RB Turbos | Quaife | Íhlins | Brembo