Since you seem to like and be able to make use the curve of a high rocker board I think you should either get the EVO or the Quad. They both thrive in the conditions you describe. The EVO 90 for sure handles a 6.4. The Quad 86 probably do too as far as the actual shape go, but I'm unsure of how the fins would handle such a big sail. There is a lot of fin in the water on the quads, but at 6.4 I have a feeling you might be overloading it a bit if you need to push it upwind at slow speeds. But I don't know this for sure. If you don't mind a bit of extra weight, a solution could be a convertible quad. Then you could have a killer big sail setup with the 16 fins in front, some tape over the rear quad boxes and a big (=16-20cm) central fin in the us box. My own testing with a thruster setup (in the quad76) has shown that it is hard to get the same combination of looseness and upwind/early planing as with teh quad setup. There is a fine line between to slippery and to much traction when you go from small to bigger center fin. But with a 6.4, the sail size will in any case not allow the quickest moves, so I think a thruster could be kind of nice. And you can still get the allroundedness of the quad for smaller sails.
Regarding the speed of the quad I actually don't know. It's for sure not something that you can measure by comparing to other sailors since you will generally be going faster by having it so much easier to go upwind. But a friend who used a fast rockered single fin , then quad and then back to single said he felt some drag on the quad, which is what you would expect. But the same guy also said he only felt this a short while into the session then didn't notice it. So it is for sure not the kind of draggy feeling that makes your whole setup feel slow and heavy. On the contrary, the Quad boards feel lively and "powered up", I think.
But then again. The EVO 90 will not disappoint.