I owned an Island Packet
cutter and this is a debate that has raged on forever. I have tried it both ways. In light wind
, use of the staysail gives immeasurable results. But in anything over 10 kts it definitely improves upwind performance, but not by much- at best a 1/2 knot
But the big advantage of a cutter is the ability to sail with the staysail and reefed main only in higher winds- about 30 kts and upwards.
If your staysail isn't setting right, and it sounds like it isn't, have a sailmaker
take a look.