Personally, Im not fond of add a leafs. They may not be the same spring rate, they may not fit in the stack very well, it changes the action of the original set of springs and the like. But it can be used and you will get some added height from it.
What is the add a leaf? Do you know its rating and what if any "lifting" its supposed to do?
I would doubt an add a leaf would add another 1.5" to that BD, and if it does it will put a strain the BD springs.