Hi again coolny29,
OK, sounds good.
Using your harness may help a bit, with early planing, but with the wind speeds you suggest, a 7.5 m2 rig is going to get you far more planing time.
Don't move back too far, or too fast.
Remember, it's a "progression".As your speed builds, you can sheet in a little more (better to keep the sail a bit more "open" at your skill level as it will have better power.
Sheeted in heavily is good for the highest speeds, by you probably aren't quite there yet so remember the old adage "When in doubt, let it out".
I think the 7.5 NCX would be an excellent addition to your quiver.
Go for it!