I was at OT yesterday and after the incident with Stuart Broad's injury, I had a look at the grille positions on my carbon fibre RAW helmet and my fibreglass old style Ayrtek helmets when I got home. Setting the grille in the same holes as Broad, it gave a gap of 50mm at the front and 55mm on the sides, which complies with the recommended maximum gaps as advised. Even allowing for the ball hitting from the side where there is less depth of peak to prevent deformation, a ball isn't going to get through.
However looking at Broad's helmet which appears to be a plastic Premiertek helmet the mounting points of the grille appear to be lower in relation to the bottom edge of the helmet. The bottom of the grille mounting plate is 9-10 mm above the bottom of the peak on the RAW, whereas on the Premiertek the bottom of the grille mounting plate is virtually level with the bottom of the helmet, which would mean Broad's grille had been adjusted to give approximately a 65mm gap, which is too large to prevent a ball entering in all circumstances.
If I was facing 85-90 mph deliveries I would definitely make sure I was wearing a carbon fibre version rather the bottom of the range helmet and I would make sure the gap in the grille wasn't too big as well, but maybe that's a reflection on my lack of confidence in my hooking abilities?