Just a quick update on this. I solved the vision issue. The problem was that my Arai Corsair V had bedded in, and it was sitting too low on my head, not that there was any issue with the lid itself.
I bought a set of Arai thicker cheek pads and a thicker helmet liner and the helmet fits perfectly now and was at the track yesterday, no problem at all with vertical field of vision.
A very cost efficient solution. (i.e. no need for a new helmet)
Cool...thanks for the update!!