I'm going next week for a couple of weeks funnily enough.
I'll be taking 400 exclusively and expect some days to have to shoot at 800 ... but you just never know down there.
The forecast is awful, and the most western part of cornwall (west penwith - Penzance, St Just, Land's End) is more like an atlantic island than mainland. The weather can just close in and stay shrouded for days if not weeks. Or be glorious. Keep an eye on the inshore waters forecast and the shipping forecast, they're more accurate for west cornwall than the ordinary one you'll get on tv and radio.