FWIW after trying all modes - one up, one down, two, etc etc, on two separate Outremers (with running backstays!) it appears that beating hard to get to a racing mark you get a better tacking angle with the two down fully. You might therefore make the mark you would otherwise miss.
If doing a long beat to windward then I suspect we go a little faster with one board down and can usually make up the lost
tacking angle so better VMG overall.
Over about 10 knots hard to windward for us there is still some gain from having some board down as the leeway definitely increases when you pull it/them up. As soon as you are slightly off the wind, heave them up.
The only way really is to do the VMG work while sailing your own boat. Then you'll realise that better sails
make more difference, etc, etc....