I visited Laguna Seca for the first time for the 8 Hours race in March, so I had no experience of it before the new fencing.
I found the track to be one of the best I have visited for photography, including at the Corkscrew. Every corner has multiple great angles. There are lots of raised areas to see/shoot over the fences - it's a very hilly place. In most places the fences aren't high anyway. I had absolutely no problems. And to answer my own question above, my 70-300mm lens was plenty. There were very few places where longer than 300mm would be useful. And the Corkscrew was mostly shot at 100-150mm zoom.
The only part of the track not accessible to spectators is the outside of the circuit from turn 6 up the straight to the outside of the Corkscrew. This means that you can only see/shoot the Corkscrew from the inside of the track.
My pics from the 8 Hours are here:
https://bit.ly/2InEow8