Hi there,
Generally you can say you need smaller sails when using a foil compared to a fin. But of course if you have a 9.0+ already, then you could probably expect to plane (with pumping) in winds well below 10knots, once you have the skills to pump both the board and sail correctly.
Planing thru lulls is exactly what the foil does so much better than the fin, but then you need to make sure you do not sail into areas with very low winds, to get going again - as a certain min is needed - or you stay up on the foil in the gybes...:-)