I don't want to kick up the old solid/pearl argument again, so I'll just say that the Ultimate uses a different formulation of the Activator cover that is more aggressive. With the same surface prep, the Ultimate is supposed to be about three to five boards stronger overall than the original Inferno. I'll be able to tell you more next week, when I get a chance to throw the two side by side.
There is the Raging Inferno, but it does have a higher RG. IMO, still a very nice ball, though.
It's possible that Brunswick will come out with one more Inferno, but from what I hear, it is highly unlikely that it'll be a higher-load particle.