They are not even close. The Sting was weaker and more down the lane than the Forza SS. The Forza SS is stronger by at least 3-4 boards for me, earlier and smoother than the Sting.
You have to keep in mind that the Sigma Sting used a pearlized coverstock, while the Forza SS is a solid cover, that's why the Sting had a faster response to friction than the SS... Though the SS is not as smoother as the original Forza, it is not as angular Sigma Sting. I would say the Forza SS fits in between the Forza and the Sigma Sting.