From what Ed Runnion has indicated before, the stock SS rear spring is one of the best from a handling point of view, but that the front SS spring is too low of a rate. When I asked him about 9C1 front springs, he felt it was not worth the effort which has made me decide to get the GWs when I get around to upgrading the suspension.
The answer to the problem of poor load carrying performance of the stock SS rears is addressed with the addition of air bags. With the Airlifts in place, I have carried some very heavy loads and still not had to air the bags up over 15 PSI. Normally I leave them at 7.5, which gives a level stance with about 100 Lbs in the trunk, or a slight rake when empty and a bit more neutral handling.