Thanks for all your suggestions.
My orange filter eats 2 stops. That would put my 1600 back down to 400.
I think it's worth investing in another 2stop ND filter to stop it further down if necessary.
I have a strong feeling that with a blue sky and white sand, even at iso 100 i'd be very close to the 1/500th shutter speed most of the time.

The GR1v stops down to f22 although I haven't shot it at that aperture much I assume it'll be fine at f22.
The only problem being it's aperture priority only.
sunny 16 for blue sky:
f16 @iso100 1/125th
f22 @iso100 1/60th
But with sand at the beach I am probably looking at something brighter than that.