Alright , it's a stupid question, but would you go faster if you had a heli following you? Only asking because I've just watched too many slalom videos!!

Depends on if it is shooting at you I guess?

No. I used to sail frequently at a lake in my area, one day there was a forest fire near by, and the choppers were flying in to tank up on water from the lake using a snorkel to suck it up, as they hovered above the water. They would fly in over our heads as we were w'surfing, in the same direction that they were coming in at. The rotor wash was far to violent and unpredictable for us to handle, we would get slapped down before the choppers even reach a straight over head position. That rotor wash would create a downward wall of wind that would block the wind that we were using. Then they would tank up far down wind from where we were sailing.
What you've been watching is shots using a telephoto lens.

But you may consider jet engine mounted on the bank.
That should work well, ( if you are Arab sheik by any chance or another happy jet owner

Yes. Provided...
(1). You cement the tail of the helicopter in the ground
(2). The rotors are pushing wind towards you, and
(3). You live in a fantasy world !