Brandon,
I think your right about the density. The fireline, like the GSP runs light. I may be wrong about this but in order to increase the mass of the leader I wrap more line around the jig pegs as I do the step down. Plus, after I wind each strand (Kathy Scott method) and let it unwind with the weight, I wind both strands together a second time to tighten the leader more. It ends up about a 15% reduction. I think the amount of winding of the leader increases the resistance to assist in the turnover of the fly. I have no math for proof of this, it just seems to make sense to me and it seems to happen when cast these leaders. I may try this on a GPS thread.
caribe