Yes, I'm spring under.I used 1.5" Rubicon Express six leaf packs on spring over. With just the tub, it was stupid tall. I could have put 40s on it. Now that its nearly complete, it squatted down maybe 4-5".
A few of the guys at WFO Concepts told me that I should expect another couple of inches. That will make the springs almost flat.
I don't know a thing about BDS springs. They may not sag, they may sag more. I doubt they make their own springs, but I could be wrong. If you have 3.5" springs, I have to assume you're sprung under.