[Errant]

You'd think they be able to model those larger pieces also.

Too many variables, beginning with the composition of the comet itself. At best, they're able to get somewhat of a useable calculation based on size, speed, distance from the sun, and repeated observations of the dust particles being emitted. And this all based on an intact nucleus. A breakup creates more surface area that is exposed and no really good way to even calculate the increased area with that model. And again, we're taking dust particles, not the big chunks dependent upon the comet's makeup.

Take another look at ISON's breakup. How do you calculate that? Look at the comet that impacted Jupiter and the one I was telling you about that has broken into about 60 pieces, the last piece coming near to earth. I think this might be it: http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=73P-C

[Yosemitest]

They were able to model this.

Discovery Channel - Miracle Planet - Large Asteroid Impact Simulation ( 9:59 )

Also there's the Downloadable images and animations of potential lunar-forming impacts (from Canup 2004, Icarus)
They CAN model those large pieces, but they just don't want to.

The formation of the Moon ( o:42 )
The formation of the Moon, Robin M. Canup (http://www.boulder.swri.edu/~robin/)
http://www.fortunadrago.it/?p=2689.