I have a 2006 A4, and a few days ago the steering boost went to full
boost even at highway speeds. The ESP idiot light also went on full-
time.

After doing some research, I found a reference to the Servotronic fuse
or controller module. This is located above the leftside dash fuse
panel, in an access hole. You can see it as the leftmost module, and
you will be able to see 'Servotronic' on the label, without taking it
out of it's socket.

Removing this fuse stops Servotronic speed-variable boost. There must
still be some boost available, as low-speed steering is still possible
even for gramma-weak drivers.

But the best news is that at driving speeds, the removal of
Servotronic boost makes steering feel incredibly better! Before, road
bumps would make me slightly move the steering wheel, and the car
would dart a bit. Also, the steering felt rubbery, as if the control
arms were mounted in very soft rubber. Both of those traits have
disappeared, and tire grip near the limit is much easier to feel. The
car tracks perfectly on-line, straight or in a corner.

I strongly urge everyone to try this mod out. If you bought an Audi
for world-class AWD handling, this makes a world of difference.