no. at extension, the only thing the shackles do is allow more travel, but they also limit the amount that it will travel. when extended, they are not "supporting" anything.
the angles are pretty much unchanged. with the scissor shackles, you have points A (body bolt) B (center bolt I.E. shackle to shackle bolt) and C (leaf spring bolt).
without them, you just have points A (body side) and "C" (aka B) of the scissor shackle system.
the distance between A and C with the scissor shackles is more or less the same as the distance between point A and B on a traditional shackle system, if you've set it up correctly. therefor it will handle the same road forces as it did with a traditional shackle (again, if it's set up properly).
now don't get me wrong...I've never done a scissor shackle system. but I have studied them and it's not exactly rocket science to figure out the geometry and weight positioning etc.