You can use the dark blue rears on the front, which are 3.5lbs softer. Keep in mind that after you adjust the preload spring to make ride height, this will make the shock pistons move higher in the body. This usually tends to make the shock lose rebound quicker (like after a full race day) and you'll need to refill the shocks more often.
Have you tried leaning the front shocks in more instead?
Sometimes when the grip is low, I actually use T2 blue springs (same length as NT1 rear springs) in the front which are another 2.5lbs softer.
