I was thinking more of the low pressure zone above the helicopter being disrupted by the overpass. Apparently it wasn't too bad since the pilot managed the stunt twice without injury, but it's a possibility that would have had me crapping diamonds if I were asked to pull it off.
So first of all, GE in a rotorcraft is different from GE on a fixed-wing aircraft. The fundamentals are the same, of course, but in a rotorcraft the relative velocity of the blade/wing isn't connected to the velocity of the airframe the way it is in a fixed-wing aircraft. In a fixed-wing aircraft the velocity of the wing and the velocity of the airframe are the same, of course. But in a rotorcraft things get very complicated. The upshot of all this complication is that the GE effect is reduced as the rotorcraft's airframe starts to move. The faster it goes, the less GE you have. So you might notice some differences while hovering under an overpass, but shooting through at 60 MPH the GE is effectively a no-op.
If you really want to know the gory details you'll have to read up on helicopter theory. It's way too complicated for an HN comment.
I think the pilot is probably correct that the stunt could only be pulled off at a higher speed where you can outrace the low/high pressure zones the blades are creating.
"I was curious about how the overpass would affect the ground effect of the helicopter. Tragically, I have no brain for physics. But I do know a helicopter pilot, so I asked him. Apparently, at 60 kts, there wouldn’t be significant ground effect because the down flowing air off of the rotor disk would mostly be tailing off of the helicopter. Had Tamburro taken the stunt slower, the down flowing air would be more directly underneath the helicopter, creating an (obviously undesirable) effect."