What is a Slipperential?

[RC-Monster Mike]

The diff is the same basic physical size as the Mugen diff, though larger in the middle where the slipper is of course. You will likely need to shim (or trim) the center diff top plate and possibly widen the spur slot in the chassis by a small amount to clear the thrust bearing - no major mods, but I expect installation in most applications may require minor work to ensure proper clearance and fit.
Weight vs. stock diff? - not sure on a direct comparison, but the complete Slipperential weighs 5.4 ounces on my scale - including the adjustment wrench, label and packaging. It has to weigh a little more than a plain diff, but it is far from heavy. :)