Stan, I'd take Al's advice. The spring rates you have are fine. The difference in the front to rear rates is because the rear springs are located furthur inward away from the wheel centerline thus changing the amount of leverage required to equal the same rate as the front spring located closer to centerline of the wheel. Fine tune with adjustable shocks and adjustable anti-sway bars. I've never had any "wacky action" in the rear due to the "exotic" semi-trailing arm rear suspension. It's was good enough for BMW.
As far as lighter spring rates, it comes down to a drivers preference. Lighter springs have some advantages and disadvanages. I'm sure others can post data on using different spring rates for different types of tracks.

Harry