the way the spring reacts in the stock shackle swing handles the load far better than flipping the shackle into a compression setup,with a flip it compresses easier and effectively makes for a softer spring rate, you get a ton more flex with a shackle flip and it gets even better if you use a longer shackle.
i have a stock rear spring with a 4" block and a shackle flip with longer shackle, i have so much flex that if i take off my shocks i can get to the end of the rti ramp, my shocks are my limiter as they do not have enough range, they are the 2nd longest ones bds makes.