I have the RSR 6.7 and use it with a X6 mast. I have found that after about 5-6 uses it will downhaul quite nicely into the max position. This is where it works best, sometimes it looks like it has to much, but on the water the performance is outstanding. I'm mid 80 kg's and have taklen it through to approx 25 knots, with no troubles so I guess depending on the water state, 30 knots would be quite realistic.
If your aim is for GPS in dead flat water, perhaps adding another .5cm downhaul to max would work, but would feel somewhat 'dead' on a reach.
In that amount of wind I would be on a 5.8 RSR if i had one!