One of the best ways to keep the slot open is to ensure that the jib has proper twist in its shape, which opens the leech up keeping the slot open from top to bottom. The best way to do this is move your cars back - this goes against some folks advice to move cars forward to open the slot. Moving cars forward opens the slot at the bottom of the sail (simple geometry), but kills the twist and closes the upper slot.

Barberhauling upwind may give a more open slot at the bottom of the sail, but it will tighten the leech which will close the slot higher up. The effect is similar to sliding your jib cars forward. You would need to find the right combo of sheeting angle to barberhauling to get a good shape with proper twist.

I've always found that when upwind speed or pointing doesn't feel right, the first thing I do is sheet the jib out a little and make sure the bows are down. You're better off with the jib slightly luffing than sheeted hard and closing off the main.

Although I'm not a 6.0 sailor, this has worked on just about anything I've sailed.