I'm gonna guess this is more accurate thermal models for motors and IGBTs so they can be pushed closer to melting point...
Elon Musk's Twitter account claims:
Model S P100D Easter egg will allow it to do 0 to 60 mph in 2.4 sec and a 10.6 sec 1/4 mile via software update next month
Putting an easter egg is imho more something for a computer game, not for a $134.000 car.
That is fun and all, but it would still lose a real race to a real sports car since it cannot sustain this power output level for over ten seconds (try it).
This gives an acceleration of 11.1 m/s2. Doesn't one need those sticky dragster tyres to get a coefficient of friction above one?