Really? I just figured set up for right around 540 the soft wouldn't be firm enough to effect the BB. Thanks for the heads up Yankee!
Quite the opposite actually. Think of it like a tire. A tire made from a softer material (ie. in terms of durometer) will grip the road better (there is a logical limit of course, as it needs to have a degree of durability or it will wear down like crazy). A bb is the same way. Soft buckings in my experience provide a more consistent spin.
I will also add that I have never been able to get the firefly to work well. Many speculate that it is because the two smaller nubs, as opposed to one, more durable one, deform too easily and inconsistently and generally lack the integrity to put a good spin on the bb that is consistent from shot to shot. Even with an arm modified to be flat as opposed to fangs, and a considerable time centering, my shots went all over the place.
Honestly, my favorite bucking is the stock TM. The stock JG actually seems to be made of the same or a very similar material, and preforms similarly.