I've had that Malowicki paper for a decade. Its very illuminating. As I recall, the solubility limit for iso-alpha acids in wort is around 80 to 85 ppm (aka: 80 to 85 IBU).
The thing that has me intrigued is the existence of another bittering compound. I can't recall its name, but its something like Humuline. Denny corrected me the last time, time for his input again. That compound is readily extracted during dry hopping and its not reflected in IBU measurement.