Speaking of the hefty digitalWrite() overhead, I noticed two things when I went poking into that code.
One, some pins are slower than others, because they have PWM timers that have to be disengaged.
Two, the "function" that turns off those timers has a really ugly chain of if/if/if/if statements that should really be a switch or at least if/else if/else if/else statements. I did a timing analysis like you, westfw, and found that the gcc compiler really does optimize the code the same way since it's all forced inline, and comparing against static const data. But it bugs me to see code that relies on the optimizer to rephrase so drastically to do the right thing-- one accidental tweak or compiler update and boom, digitalWrite() could be twice as slow, because the compiler decides to implement what is written, not what might possibly be implied.