Depends on the board. If the original design was meant to be convertible, they can do okay. If not, then, as LeeD says, the foil optimizations impact their performance.
My first foil boards were convertibles - respectable fin performance but then I gave up a bit on the foiling side for that (strap placement, tail width, etc.)
I sailed a Levitator 160 with the 61cm mast and no foil. Very bouncy on a plane, and slow, but stable and would be a fine board for teaching absolute windsurfing beginners.
I think this? They look like they are planing okay but as someone said,a bit slow and bouncy. Not sure they plane well through a jibe,either. btw, the guy in front is a pretty good foiler now.
The Stingray 140 with a formula fin cut down to 50cm is not bad. It has rails that are sharp enough to work OK for finning. The rails also make it easy to pump for foiling.
The Exocet 132 is horrible for finning. Its rails are basically nonexistent. Completely round from top to bottom. Great for foiling, awful for finning. Even for foiling, this board requires aggressive pumping until you get flight. The rails don't help for pumping or for slogging upwind, but they are nice and soft for slap-downs. This is a complete foilboard and nothing else.
I've used my JP 120 Foil Board with a 45cm deep tuttle fin. It worked pretty well both in the ocean and on flat water. It was still quite fast and qybed fine.
