hmmm... so the track is making contact with the ground at the front of the rails? is this what you are saying?... How much travel is being used up on the ski shocks when the sled is just sitting there? When you lift the front of the sled, how much shock travel do you see coming back?
Anyway, what should happen with the center spring adjustment is that you should typically get more weight transfer when the sled is under acceleration.
When you adjusted your rear spring what you did was actually INCREASE ski pressure by forcing the back end of the sled up, you forced the front end down, so adjusting your center spring brings some of the weight transfer back, and if you adjusted your ski suspension, you would be forcing the front of the sled up and the rear down. See what I'm getting at???
So to answer your last question, YES... this will force your sled to lean more to the back when you hit the gas, causing your skiis to get lighter.
What you mostly want to do is the ski shocks and centre spring... Unless I'm missing a point stated in your original post?