A lot of wise and knowledgable men have spoken in this thread. Aerodynamics is certainly key in going fast. Least amount of drag with the right amount of downforce. LSR fairings do not tolerate hardly any crosswind, so leave the bike in the trailer if the winds are not favorable.
Horsepower is not an issue. One phonecall to Richard, Sean or Seb (in no particular order) and you will have more than enough to go 300+ in the mile or 311+ in the 1.5. It is all the other things that need to be right. Correct wheelbase, swing arm length, weight distribution, CG etc etc. Then comes the analysis of the data logging so maximum HP for every segment can be applied during the entire run.
Large Formula 1 team use to have two wind tunnels ((different scale cars) operating 24/7 before the rules clamped down on that in the interest of cost. They also use CFD extensively to simulate different conditions, including sideslip and crosswind. They can tell with fairly good accuracy how new bits on the car will behave on the track. But they have armies of aerodynamicists, many with PHDs and still don't always get it right mostly due to calibration errors. The bottom line is; even a well tested fairing may not perform as expected during actual use. Trial and error. Bill, as described earlier in this thread, tirelessly pursued perfection in all areas mentioned above.
Records will always be broken; it is just a matter of time. Maybe Bill's speeds will be exceeded in our lifetime, maybe not. The bar is high.